Well... it will improve CPU performance in a manner of speaking, because it has less overhead than Direct3D or OpenGL. This will benefit Intel processors too (when equipped with GCN discrete graphics). However, it may actually help AMD more (as a percentage) since they tend to have lower IPC, and thus Mantle helps reduce per-thread CPU bottlenecks. I bet this will provide quite a large boost to GCN-equipped APU-only setups quite a bit (especially mobile), since it somewhat mitigates the need for faster CPU cores. How big of an impact this will have depends on how many engines support it. Frostbite is a really big win, however.