Mob: I think you oversimplify a complex issue.
Processing power. By this I assume you actually mean graphics processing. This point is entirely dependent on the coding. Blizzard are one of the best game makers when it comes to coding to mac, whilst they do occasionally make mac specific errors they also code specifically for the platform, supporting OpenGL etc etc, and having long term influence to some extent on the platform OpenGL development within Apple. Your comment would hold some truth if you were talking about poorly ported windows games or even worse, Directx games being emulated through Transgamings Cider solution. In that case yeah there is a pretty high processing overhead.
The second issue is specs, not all macs are made the same. I have a 6970M card in my 27" machine and it can handle pretty much anything you throw at it with ultra settings @1080P. This includes things like Battlefield 3 on bootcamp. However I hate running at 1080p and much prefer slightly or moderately lower graphics settings at 1440P. Much crisper and clearer. Blizzard games run either at ultra or very very close (SSAO on Mists of Panderia being the one problem) on native resolution on this machine. Point being that the required resolution is an important factor.
If you are using the lower resolution screen I would think you would be able run D3 on at least mid settings. I would more careful about recommending D3 at native resolution 1440P (27") though at mid settings on that card.