Killzone 2 pushes the PS3 far more than Red Faction and as the devs told us they are nowhere near to maxing out the PS3 hardware.
Of course there are going to be headaches to deal with when designing a 360/PS3 multi-platform game engine, because despite some similarities they are too different to take optimal advantage of.
For the 360 it does not matter if this was an exclusive, for the PS3 it does due to its innovative nature.