The 1:1 straight-line behavior of fullscreen post processing is much easier to prefetch than triangle rasterization. And, in this case the SPUs and GPU used the

The 1:1 straight-line behavior of fullscreen post processing is much easier to prefetch than triangle rasterization. And, in this case the SPUs and GPU used the same memory. So, no bandwidth advantage to the GPU. The best the GPU could do would be hiding latency better.