You have to keep in mind that such tweaks may be required on a 'per level' basis, so its a lot of man hours to optimise performance, if a console is struggling to run your engine.
(EG, an ice level would use different hardware resources than a tropical water filled level)
I have to assume that the SPEs are programmed on an individual basis. That is, the developer has to tell it to put this thread on that SPE. Why it is a PITA is because the workload isn't balanced by the OS. In Windows, all you do is something like this...
Code:
for (int i = 0; i < Environment.ProcessorCount; i++)
new Thread(MyWork).Start(stack[i]);
Viola, you now have as many cores as you have available working on different stacks of work. Windows will put that thread on the core with the smallest workload. Which core is which is transparent/meaningless to the developer. On PS3, I assume it is more like this...
Code:
for (int i = 0; SpeCount; i++)
Spe[i] = new Thread(MyWork).Start(stack[i]);
On the surface, this looks pretty much the same but you have to realize that every stack takes different amounts of time to process. Because games are naturally symmetrical (all tasks must be completed for this frame before work can start on the next) and not asymmetrical (work is done as fast as it is done with little impact on the entire scheme of things), one overburdened SPE could mean a sizable drop in framerate.
On Windows, you "fix" this issue by having more than "ProcessorCount" threads. This gives Windows the option to take several lighter threads and separate them on a core that doesn't have a single heavy thread running on it. Windows will fix the problem for you.
On PS3 (assumed), you fix the problem by coding your own version of what Windows does--find what tasks are overburdeneing a SPE and split/move it to another thread that doesn't have as much burden.
Ultimately (again, assumed), SPEs don't have the control over its own micromanagement that we see in fully integrated cores. It adds more burden (in the form of micromanagement) on to the developers and they, therefore, won't address the issue unless it is deemed absolutely necessary (like Carmack with RAGE--he is having to address the SPEs individually in order to get the performance at least on par with Xbox360).
It has nothing to do with "levels" and everything to do with load balancing across the available hardware. Ice levels most likely use a glass-esque DX9 render material while a tropical level would use a lot of water DX9 render materials. Both have a larger impact on the GPU than the CPU. The CPU usually deals in unit statistics, physics, AI, audio (if there isn't a dedicated sound device), and pretty much everything else that isn't visual.
