A wild guess would be that OS X drivers contain poor shader optimizer.

Parallels probably work by decompiling DirectX shader bytecode back into GLSL, which is already optimized for OS X shader compiler, therefore there is less burden on the optimizer.

Another example of poor drivers are various GLES drivers on mobile, mostly trivial optimizations can drastically increase the performance: http://aras-p.info/blog/2010/09/29/glsl-optimizer/ and https://github.com/aras-p/glsl-optimizer