regardless of whether or not the game actually utilizes 100% of the PS3's power, FF games, especially ones that first come out for a console, always were graphically superior and overall just great games [FFVII, FFX]. this means that FFXIII should not disappoint, even if it doesnt use up all the PS3's power. and if it does, that doesnt mean that games wont get any better, it just means that they wont look as great as FFXIII promises to be.
either way, on the topic of a 360 port, i highly doubt that it will actually be ported, especially if it does use 100% of PS3. if it is ported, who is gonna want to buy a game that they have to change 5 discs to play?? this is a reversion to the old days [PS1,PS2]. one of the best attributes of next-gen gaming is that the storage media is sooo massive that they shouldnt even be making games that need to change discs.