Would it really sell more on 360? You're looking at a franchise that has been exclusive to Sony systems for all four of its existing installments, two of which have been exclusive to PS3. Any franchise with a history like that would probably sell more on its original platform even if it were released on other platforms, regardless of genre. Games like Call of Duty perform somewhat closely on both platforms as it is.
Indeed. It would be a very Final Fantasy-esque scenario, where it would always do better on PlayStation.
Well, Final Fantasy spans generations and has the advantage of Japan.
I'm sure that, given enough time, the Killzone franchise could start selling better on other platforms should it go multiplat. However, it would probably take a number of iterations and probably a generational leap to do so.