I agree - after writing my comment, reading the other bits of the thread made me think that "matches internal experience" probably correlates with "won't regret transition". It can be worth having norms that reduce the incidence of people making choices they'll regret later on.

The best approach to the problem might be to look at the experiences of those that have transitioned, and see which factors predict a greater improvement in happiness - at least, that would seem more conclusive to me than reasoning about subjective experience and emotion being more or less like that of the destination gender (though as you say, that has it's importance too).

(By the way, I'm not sure I see what Postel's law has to do with this - "whooosh", as they say)