I didn't need a shim on the CPU. As seen in your pictures, the CPU is making contact with the heatsink, while the GPU isn't... it's evident because the compound isn't flattened like it is on the CPU side. The compound itself is the only thing that makes the GPU come in contact with the heatsink. Clean all the goop off, and take the heatsink out of the computer. Mount the heatsink to the board using the screws that go around the CPU/GPU. Then measure the gap between the GPU die and the heatsink. If it's 0.8mm (which it most likely is), then this is proof enough that everyone with a PowerBook G4 that uses this heatsink setup needs to shim the GPU.