This comes straight from Insomniac's brochure released on 1ts January,2008.
SPUs
All our systems started off as RAW mode. The only long term (not finished this frame) asynchronous processing is the collision on the raw SPUs. We use [Job Manager] but not all systems use it in the typical way of fire and forget. Most of our system require the SPU to be running a particular system at the same time as the PPU. To ensure the SPU is doing what we want at the correct time we send [Job Manager] the job and use our own thin synchronization and job buffering schemes using the locked-line for communication
10-20% total SPU utilization