I don't know of any major differences between Japanese and US versions of any FF games, besides FF4 which apparently was made a lot easier in the US version (I haven't played the US version, only the Japanese one.. and FF Anthology, but that's based on the Japanese versions as well).
And having ROMs doesn't count as owning the games.. owning the games is having the original cartridges or the PS1 re-releases. So I own all FF games up to X besides FF3j (have that on emulator, after all, it's the only way to play it still), and not going to get XI
