This puzzles me. Last I heard, the Mac versions of Valve's games were slower than their Windows counterparts. So maybe it's more about the Linux kernel than about OpenGL? Is there any comparison between the three platforms on the same hardware?
Source: Current version on newest OS X if you don't want to support Legacy OS X is 3.2, Legacy OS X is 2.1 https://developer.apple.com/graphicsimaging/opengl/capabilit...
Most recent OpenGL is 4.2. http://www.opengl.org/
Most OS X coders, till the recent OS X release have been using 2.1 which was released in 2006 which is when OpenGL was inferior to DirectX in every way.
That's in part a driver issue (though Mach message traffic may also limit perfs), the drivers are mostly developed in-house by Apple, and Apple is notoriously hard to work with (for third parties). So while Valve did lead to significant performance improvements following Steam/OSX I would expect they couldn't work closely with internal drivers developers.
And game perfs probably still isn't the main focus for OSX drivers development, sadly.
Really? Are you sure about that? I'm pretty sure that while there's some Apple-supplied kernel-mode glue, and maybe a lot of user-space components, most of the driver code is actually supplied by NVidia/AMD/Intel.
In fact at one time or another OSX seems to have inherited funny GPU driver bugs that were common with Windows (like the way NVidia's 32-bit drivers were unhappy with living outside the 2GB-4GB memory range).
Although these days I'm it looks like they write significant portions of the graphic driver subsystems: when released their automatic graphics switching seemed quite markedly different from Optimus and completely independent[0], which is corroborated by them not wanting to restrict their ability to switch back to AMD.
[0] http://arstechnica.com/apple/2010/04/inside-apples-automatic...