Someone made a claim that the GPU included with the 13inch model is close to the 9400M - the reality is anything but that. Benchmarks that were leaked a while ago showed that at the worse case scenario it was equal to the 320/330. The only thing that will let down the GPU isn't the hardware but whether the driver and OpenGL stack are properly optimised. Given how close Lion is to being released (in the next 6 months) I wonder whether the focus has been more on getting Lion optimised than spending a tonne of time on optimising Snow Leopard.
the gpu in the 13" mbp should only be in a netbook and not a mbp for the price