My statement was vague. What I meant to write was that Mac screens generally have high resolutions paired with less powerful resolutions. If I were to have a 21.5" screen for a PC/Windows build, I would not pair it with a 6750m card. So I'm curious as to whether an iMac with a 6750m can really handle Diablo III on a 21.5" screen with the resolution maxed.
The iMac screen resolution is listed as 1920x1080. The 6750m should be able to run Diablo3 at that resolution at decent frame rates (40+ avg.) on medium settings.
Edit: Blizzard allows you to download either client after purchasing the game so I suppose if the Mac version in gimped in someway you could just download the PC version and run it in bootcamp. I am not sure how they would have better optimization on the Mac version to justify the lower system requirements but maybe they do.