I'm a little skeptical about this: "Attendees will be surrounded by other ambitious, successful, practically-minded folks"

Can I get some evidence?

Presumably that depends on how we came to think we held that moral theory in the first place.

If I assert moral theory X because it does the best job of reflecting my moral intuitions, for example, then when I discover that my moral intuitions in a particular case contradict X, it makes sense to amend X to better reflect my moral intuitions.

That said, I certainly agree that if I assert X for some reason unrelated to my moral intuitions, then modifying X based on my moral intuitions is a very questionable move.

It sounds like you're presuming that the latter is generally the case when people assert utilitarianism?

Well if you view moral theories as if they were scientific hypothesis, you could reason in the following way: If a moral theory/hypothesis makes a counter intuitive prediction you could 1) reject the your intuition or 2) reject the hypothesis ("I want to") 3) revise your hypothesis.

It would be practical if one could actually try out an moral theory, but I don't see how one could go about doing that. . .

Here's his 2008 paper, "Life Extension versus Replacement," which explores an amendment to utilitarianism that would allow us to prefer Life Extension

I feel like the thing that should allow us to prefer life extension is the thing that makes people search for amendments to utilitarianism that would allow us to prefer life extension.

A currently living person doesn't want to die, but a potentially living person doesn't yet want to live, so there's an asymmetry between the two scenarios.

If you want impact, use the narrative fallacy. What I mean is, use all of the other biases and fallacies you listed - tell a story about John, the guy who met a cool scientist guy when he was in primary school and now his life goal is to be a scientist. He decides to do work on global warming because 'what could be more important than this issue?' He expects to live in the city, be the head of a big lab... But he's not very good at global warming science (maybe he's not very good at research?), and he doesn't seem to notice that the advice his colleagues give him isn't helping. So he sticks to his guns because he's already got a degree in global warming, but he's always stressing about not having a job...

And so on.

And then rewind. John discovers rationality when he's a young adult, and becomes John-prime. Compare John to John-prime, whose rationality training allows him to recognise the availability bias at work on his dream of being a scientist, and since scholarship is a virtue, he researches, interviews... discovers that politics is a much better fit! His rationality informs him that the most important thing is improving quality of life, not global warming or power, so he donates to third-world charities and ensures when he runs for political positions he does so on a platform of improving social welfare and medical access. His rationality lets him evaluate advice-givers, and he manages to see through most of the self-serving advice - and when he finds a mentor who seems genuine, he sticks to that mentor, improving his success in politics...

And so on.

(And then the punchline: explain why this story makes the audience feel like rationality is important with a description of the narrative bias!)

Why does one have to be better than the other?

I really need to fix my blog archive, but I discussed this in the post at the top of this page.

Please don't do that. OP's comment doesn't make any sense now.

response a) My life gets better with each year I live. I learn new things and make new friends. 2 people who live 12 years will not have the same amount of happiness as I will on my birthday, when I turn 24. I see no reason why the same should not hold for even longer lifespans.

Response b) I privilege people that already exist over people who do not exist. A person living 800 years is more valuable to me EVEN if you say the same amount of happiness happens in both cases. I care about existing people being happy, and about not creating sad people, but I don't particularly care about creating new happy entities unless it's necessary for the perpetuation of humanity, which is something I value.

response c) The personal response, I value my own happiness significantly higher than that of other people. 1 year of my own life is worth more to me than 1 year of someone else's life. If my decision was between creating 10 people as happy as I am or making myself 10 times happier, I will make myself 10 times happier.

Finally, you don't seem to realize what is meant by caring about average utility. In your scenario, the TOTAL years lived remains the same in both cases, but the AVERAGE utility goes far down in the second case. 80 years per person is a lot less than 800 years per person.