@ 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)