What are you trying to do that is disappointing you? After Effects doesn't use the GPU for much. The only feature that really used the GPU very intensively (the ray-traced renderer) is considered obsolete. (I don't really know anybody who used it professionally.) Now, some third-party plugins (like Element, Red Giant Universe, etc.) use the GPU, but they have nothing to do with AE's ray-traced renderer and they will use the Mac Pro GPUs.