It's just a guess, given how big the game is likely to be and how much is going on. The framerate in SR4 dipped to the point where the game would literally stop at times on 360, and that game, while visually pretty busy, did not appear to be at all the scope of this one. Framerate on PS3 as I understand was fine, and on PC the game was gorgeous.
Huh? The game can run like doo doo regardless of the architecture. See my example of SR4. Or hell, look at ANY Bethesda game on the PS3. Or Far Cry 3 for the 360. Or Bayonetta for the PS3. I hope that I'm wrong, because it sucks when both versions of the game aren't great, because inevitably someone is going to not have the best experience. But to act like it doesn't happen is just stupid, and to act like the 360 isn't already some seriously aging hardware is stupider.