@MikeB: For me, the real interesting question (and the one we'll probably not get an answer for) is: What is limiting KZ2's performance as it stands? Very likely there is one element of the system that they ran into, as it happens with any advanced game which tries to push the hardware (i.e. mostly games from technically excellent teams).

Note the "as it stands" part, I'm not talking about hypothetical advantages of the PS3 architecture that they haven't taken advantage of. Talking about unrealized improvements doesn't answer my question in the least. If they're using the GPU fully, those hypothetical improvements haven't yet come into play to negate that.

My point is that the only thing we know is that it's not a lack of SPU cycles, as they aren't fully using the Cell. Maybe the GPU is being nearly fully used, or something else.

@ NJ5

Well I guess memory, but that's a moving target as well with endless potential code optimisation (resulting in smaller footprints), the potential to use more procedural generated graphics (resulting in far less storage requirements for assets, something the SPUs are really powerful for), OS optimisation, harddrive virtual memory / streaming, bandwidth optimisations, etc.

You will never get a concrete answer as it's not possible. Current CPU load can be easily determined through Sonys benchmarking tools. That's the only reason why we get figures on those.

PS3 exclusive Genji 2 - 0% max SPU usage.
PS3 exclusive Resistance: Fall of Man ~16% max SPU usage.
PS3 exclusive Uncharted: Drake's Fortune ~30% SPU usage
PS3 exclusive Killzone 2 (current build) max ~60% SPU usage per SPU (average will be lower)

@MikeB: A concrete answer is possible, even if they don't give it to us. I can assure you that some people at Guerrilla Games know fairly well how much KZ2 is using each piece of hardware. It's very important to have that data when optimizing software.

There are also GPU profilers, but even if there isn't one for the PS3 the engine guys must have some data on that.