"Is it possible OSX just has more overhead?"

Most likely, it's because Apple hasn't cared much about 3D performance in the past -- and even if they did, they'd be behind, because nVidia and ATI can dedicate entire teams of people specifically to opimizing drivers. Apple really can't do that, because Apple makes operating systems and consumer electronics, not graphics processors.

And as the graphics processors get more powerful and evolve toward being ever more general-purpose, the driver gap between the specialists (ATI, nVidia, and eventually probably the Larrabee team at Intel) and the likes of Apple will only grow.

And the case in point -- Microsoft provides reference drivers, nVidia and ATI provide the real deal... those highly optimized drivers aren't Microsoft's code.