I love genderswap because I love seeing more women in major roles that they don't often end up in in fiction. I also love, as you said, seeing how oppression based on gender may have affected the character and how it may have changed their outlook on life and personality. But more than that, as gender, and gender oppression is only a small part of a person's identity and general make-up, I like seeing how much of the character's personality is still the same. How they react similarly, who and what they are drawn to, what they value and deem important. Often my favourite stories are ones where the character's personality is mostly the same but the world reacts differently to them based on (usually oppressive) beliefs about gender.

I think Finn/Fionna would be pretty much exactly the same person, assuming Stormtrooper training isn't much different for men and women. I don't think Rey would be any different, or Poe for that matter, considering there are male and female pilots, and the Resistance seems more egalitarian than most places. I think a male Leia would have had an easier time of it than female Leia actually had, in terms of rising to the top, persuading the Republic of the danger of the First Order and so on. There definitely still seems to be some sexism in the Star Wars universe, but perhaps not to the extent that there is in the real world.