No, FFXIII will not be downgraded like GTAIV.

[Staude]

starcraft said:

Now clearly the issues of disc space that may or may not have existed for GTAIV don't exist for FFXIII due to it's linear and therefore multiple-disk nature, but a lot of people, including RED53DEVILS, still fear that the PS3's memory weaknesses will result in the "downgrading" of the Xbox 360 version to match the potentially poorer performance of the PS3 edition.

 

You should rather wonder if the game will be gimped because of the weak cpu of the 360 rather than the memory of the ps3. Which is infact the same amount of ram of which half performs at twice the speed. It might not be shared memory, but it's better memory.