Apple makes very efficient processors make now doubt about it but the point is a majority of dev's don't care. PC's reign in this this field and as long as the price of MAC's stay in the sky, I can't see them gaining any large percentage of market share in the foreseeable future.
D3 will most likely running excellent on the XBOX (30fps) but at the expense of image/texture quality. The focus on optimizing games for the MAC platform is quickly being replaced by the increased demand for Linux support.
Also based up on ID's developer relations, D3 will mostly run the best on an Nvidia/PC based platform.
btw, Sat I have that issue of Maximum PC and can confirm the G5 did get its butt spanked in most apps compared to its AMD & Intel counterparts.