I would guess that most people who are serious enough about these types of questions to be involved in animal EA would probably distinguish between different animals.  I feel like after reading about (sorry, I'm not going to take the time to dig up the sources) the subject and talking to some people in the EA community, my views are now approximately:

-80% confident factory-farmed, caged, chickens are net negative (agree they're less similar to humans, but the conditions are so bad, that the physical pain alone seems very bad)

-70% confident factory-farmed pigs are net negative (better conditions than chickens, but seem more likely to be bothered emotionally by being in captivity)

-70% confident factory-farmed cattle are net positive (probably depends on the farm and how much time they spend on a grass diet)

By net positive/negative, I mean just for the animal itself, not for the world, which might require health and environmental concerns, though I tend to think animal welfare is the largest factor in most cases.  Also, confidence that their lives are net positive or negative does not really matter as much as the distribution of how positive or negative, but obviously that would be a lot harder to communicate quickly here.

I assume most other birds are treated like chickens, though I don't really know. I have not given enough thought to fish.

For reference, I'm only like 80% sure that the average human life is net positive.  It just seems like a really hard thing to know, but that's a conversation for another time.  I'm just stating this for calibration.

10%: Sentience-maximizing diets. If you think animals in factory farms have net-positive lives, then it makes sense to want to increase the number of animals (by eating the most meat-heavy healthy diet possible) while also working to improve their welfare.

You made a great point for canibalism.

In the "factory farmed animals have positive lives" camp, I'd imagine that people aren't going to think that those lives make for particularly cost-effective ways to generate hedons. Maybe I'm wrong but I'd imagine most people who consider these lives positive would say they're weakly positive. By contrast, among the people who consider these lives negative, many consider them highly negative. Most people don't actually care about maximizing sentience per se (though some people might); instead, it's about the quality.

I'm not sure what points can actually be made by this line of reasoning.  What I mean is taking the naive view throws out any useful information and leaves us with the status quo.

And you conclude with boycott-itarianism + the status quo essentially with each side cancelling each other out in all other subgroups.  The boycott-itarianism addition comes mainly from the assumption of utilitarianism (since we're talking about EA and all).

Let's use this reasoning in a different situation e.g. human population.

Whether a human life is net negative or net positive seems like a very difficult question. So naively there should be a 50/50 split (minus any arguments for a better prior).

50% we should reduce the human population

50% we should increase the human population

we can have similar splits

with each side having its die hards (euthanasia vs octo-moms/dads) 

handshakes ("I'll kill one less person if you have one less kid" and vice versa)

welfarists (we should increase the welfare of current humans)

anything (normal 2.5 kids)

which adds up to the status quo + the welfarists.

it'll always be the status quo + the moderates if you give them equal splits and assume the utilitarian position at the offset.

This seems like a super hard question, and not one that changes the importance of working to promote animal welfare, so naively (absent some argument for a more informative prior) it should have a 50/50 split within animal welfare circles.

My intuition is that many people are drawn into animal welfare specifically because they think that factory farming is clearly net-negative, thus making this an important cause area, while people who think that factory farming is positive are likely to see other EA cause areas as more urgent and gravitate towards them.

Within the "net-negative" camp, in my unanchored "what would I naively expect?" hypothetical, I then imagine dietary preferences breaking down something like:

• 10%: Approximate veg*nism or approximate reducetarianism. ("Approximate" to allow for carve-outs like bivalves and especially-moral animal products. The group generally strongly encourages all members to have at least one carve-out, because bivalves in particular are such a clear case and dietary purity ethics is a risky attractor to avoid.) 
• 10%: Handshake-itarianism. [...]

Within the "net-positive" camp, I imagine:

• 10%: Sentience-maximizing diets. If you think animals in factory farms have net-positive lives, then it makes sense to want to increase the number of animals (by eating the most meat-heavy healthy diet possible) while also working to improve their welfare. 
• 10%: Handshake-itarianism. [...]

Handshake-itarianism observes that the ~veg*ns and the sentience-maximizers are sort of offsetting each other's efforts, and that it can make more sense for Bob the ~Veg*n and Alice the Sentience-Maximizer to pair off and each agree to eat a "compromise" diet (e.g., both eat meat but only on the weekend). 

Sentience-maximizing diets would make logical sense, but I don't think I recall ever meeting anyone (EA or otherwise) who would follow such a diet, while I have met plenty of people (EA or otherwise) who follow approximate veg*nism or approximate reductarianism. For that reason I'd also be surprised if both camps really had the same numbers of Handshake-itarians, simply because there seem to me to be vastly fewer sentience-maximizers to trade with than there are ~veg*ns.

See also the discussion on the EA Forum post.

After looking at the evidence, I think conventional factory farmed chickens (for eggs and meat) have net negative lives in expectation (on symmetric ethical views). See my thread.

Note that there might be other crucial factors in assessing whether 'more factory farming' or 'less factory farming' is good on net — e.g., the effect on wild animals, including indirect effects like 'factory farming changes the global climate, which changes various ecosystems around the world, which increases/decreases the population of various species (or changes what their lives are like)'.

It then matters a lot how likely various wild animal species are to be moral patients, whether their lives tend to be 'worse than death' vs. 'better than death', etc.

And regarding:

The number would be much higher than 60% on strictly utilitarian grounds, but humans aren't strict utilitarians and it makes sense for people working hard on improving animal lives to develop strong feelings about their own personal relationship to factory farming, or to want to self-signal their commitment in some fashion.

I do think that most of EA's distinctive moral views are best understood as 'moves in the direction of utilitarianism' relative to the typical layperson's moral intuitions. This is interesting because utilitarianism seems false as a general theory of human value (e.g., I don't reflectively endorse being perfectly morally impartial between my family and a stranger). But utilitarianism seems to get one important core thing right, which is 'when the stakes are sufficiently high and there aren't complicating factors, you should definitely be impartial, consequentialist, scope-sensitive, etc. in your high-impact decisions'; the weird features of EA morality seem to mostly be about emulating impartial benevolent maximization in this specific way, without endorsing utilitarianism as a whole.

Like, an interest in human challenge trials is a very recognizably ‘EA-moral-orientation’ thing to do, even though it’s not a thing EAs have traditionally cared about — and that’s because it’s thinking seriously, quantitatively, and consistently about costs and benefits, it’s consequentialist, it’s impartially trying to improve welfare, etc.

There’s a general, very simple and unified thread running through all of these moral divergences AFAICT, and it’s something like ‘when choices are simultaneously low-effort enough and high-impact enough, and don’t involve severe obvious violations of ordinary interpersonal ethics like "don’t murder", utilitarianism gets the right answer’. And I think this is because ‘impartially maximize welfare’ is itself a simple idea, and an incredibly crucial part of human morality.

There is also the slightly odd perspective that starts by saying 2 computers running the same computation only morally count once, and then goes on to claim that 2 battery hens are so mentally similar as to count as the same mind morally.

I think this misses opportunity costs. If farm animals have net positive lives, then accidentally filling the universe with animal farms would be better than accidentally filling it with paperclips. But we might want to use the universe for something else.

Accordingly, the fork in this naive imaginary community of naive idealized utilitarians shouldn't be whether people think farm animals have net-positive or net-negative lives. It should be whether those lives are better or worse than the lives that would otherwise be supported by those resources, plus the improvement or degradation in lives that already exist, on the margin.

For example:

• Alice thinks that farm animals have net negative lives, but farming mostly competes with wild animal habitat, and wild animals have worse lives than farm animals. So she eats more meat, on the margin.
• Bob thinks that farm animals have net positive lives, but farming mostly competes with human habitat, and humans have better lives than farm animals. So he eats less meat, on the margin.

I wonder if that alters the 50/50 split imagined here? Evolution isn't interested wild animal welfare, and farming isn't interested in farm animal welfare. But humans are very interested in human welfare. So in our naive thought experiment, transferring resources from farm use to human use will be net positive for welfare, not a 50-50 coin toss.

First, this assumes total utilitarianism. While I don't fully endorse any kind of utilitarianism, average utilitarianism is more appropriate for this purpose IMO (i.e reflects our intrinsic preferences better). I want the world at large to be nicer, not to contain as many minds as possible. I doubt anyone cares that much whether there is one zillion or two zillion minds out there, these numbers don't mean much to a person. (And, no, I don't think it's a "bias".) And, it seems quite plausible that factory farmed lives are below average. Moreover, the close association of factory farming to human civilization makes the situation worse (because the average is actually weighted by some kind of "distance"). To put it simply, factory farming is an ugly, incredibly cruel thing and I don't want it to exist, much less to exist anywhere in my "vicinity".

Second, I don't understand the statement "EA is generally about optimizing your positive impact on the world, not about purifying your personal actions of any possible negative impact." I'm guessing that you're using a model where a person has some limited number of "spoons" for altruistic deeds, so spending spoons on veganism takes them away from other things. This does seem like a popular model in EA, but I also think it's entirely fake. The reality is, we do a limited number of altruistic deeds because we are just not that altruistic.

If judged by intrinsic preferences alone, then plausibly the tradeoff between selfish and altruistic preferences is s.t. going vegan is not worth it individually but worth it as a society. The reason people go vegan anyway is probably signaling (i.e. reputational gain). And, signaling is a good thing! Signaling is the only tool we have to overcome tragedies of the commons, like this one. The role of EA should be, IMO, precisely creating norms that incentivize behavior which makes the world better. Hence, I want EA to award reputation points for veganism.

I imagine the ultimate solution to non-human welfare as follows:

• (Trans)humans do not kill any life to survive. This include lab-grown unicellular life. All transhumans are mind uploads (ems) whose subsistence needs are fulfilled by electricity and raw materials to build new hardware
• All the agriculture and heavy industry is located in space. The entire Earth is preserved as a wildlife park   
• All death is eliminated, human and non-human:
  • Smart-enough non-human life (chimps etc) is uplifted and uploaded
  • At the earliest possible opportunity, the rest of the biosphere is cryopreserved (or placed in a stasis by other means), to stop all death, "natural" and anthropogenic. We decide what to do with the biosphere after we got enough compute for such a complex decision

From this point of view:

• mind uploading research could be the most efficient long-term animal welfare measure
• asteroid mining might be the second most efficient measure, as it is necessary for moving all industry to space 
• the most ethical dietary choices are the choices that help with the transition to the described future:
  • if your research helps with the transition, eat healthy food that prolongs your life (e.g a lot of greens and almost no meat). The longer you live, the more you can contribute to the research
  • in general, reduce the consumption of cattle-derived food, as it indirectly slows down the transition (cattle -> global warming -> negative economical impacts -> less money for the relevant research)

Your Boycott-itarianism could work just through market signals. As long as your diet makes you purchase less high-cruelty food and more low-cruelty food, you'll increase the average welfare of farm animals, right? Choosing a simple threshold and telling everyone about it is additionally useful for coordination and maybe sending farmers non-market signals, if you believe those work.

If you really want the diet to be robustly good with respect to the question of whether farm animals' lives are net-positive, you'd want to tune the threshold so as not to change the number of animals consumed (per person per year, compared to a default diet, over the whole community). One would have to estimate price elasticities and dig into the details of "cage-free", etc.

There is another approach that says something along the line of not all farm-factories animals have the same treatment, for example the median cow is treated way better than the median chicken, I for one would have to guess that cows are net positive, and chickens are probably net negative (and probably even have worse lives than wild animals)

The number would be much higher than 60% on strictly utilitarian grounds

Why? 

In most circumstances any given act of (non-)consumption is unlikely to make a difference -- but aggregate present consumption must play a large role in determining future production.

My default assumption is that each decision not to consume 1 animal's worth of meat has a 1/n chance of preventing the raising for slaughter of n animals. That can only be an approximation of the truth, but what is a more reasonable estimate? 

Or if that is agreed to be a reasonable estimate, why should a utilitarian who cares about animal welfare, and believes that agricultural animals' lives are net negative, not change their diet as a result?