TL;DR version: Yes, Diablo III runs better in Windows than in Mac OS X.
I have a Mid 2011, 27" iMac (Specifically, with a Core i5, 2.6GHz CPU and a Radeon HD 6770M graphics card). I installed Diablo 3 in both OS X and Windows 7 and ran a few simple tests by loading up a character who was parked at the Act II starting area.
The default/suggested video settings were the same on both platforms:
- 1920 × 1080 resolution
- High texture quality
- Medium shadow quality
- High physics
- High clutter density
- Anti-aliasing on
My 27" iMac's display has a native resolution of 2560 × 1440. I did a separate set of tests at this resolution as well.
Changing the texture, shadow, physics, and clutter settings only had minor effects (at least on my machine), but I did notice a significant boost by disabling anti-aliasing and turning on the "Low FX" mode.
(In the chart below, D means default settings, L means default settings with Low FX mode enabled and anti-aliasing off.)
1920× D 1920× L 2560× D 2560× L
OS X: 34 FPS 39 FPS 23 FPS 27 FPS
Windows: 47 FPS 53 FPS 27 FPS 35 FPS
Improvement: ~38% ~35% ~17% ~29%
I didn't exhaustively test every combination of settings and resolutions, but based on these results, I'm willing to state that yes, Diablo III runs better in Windows.