I think that the argument also holds on a per-unit of meat basis (therefore controlling for the quantity that humans eat), although you only get a factor of 20 for chickens vs cows. (Here is source, warning that you have to scroll through pictures of chickens and fish being tortured to get to the numbers). I also sort of agree with the case for ostroveganism, although I haven't thought about it much since I find the thought of eating seafood viscerally disgusting anyway.

I agree that we should think that animals further away from us evolutionarily are less likely to be sentient and suffer, but I'm not sure that the drop is as big as a factor of 10.