When starting with the OpenArena game, which are more CPU limited and not that all demanding by modern OpenGL drivers, Ubuntu 11.10 and Mac OS X 10.7.2 are at nearly the same speeds with an "out of the box" configuration. Mac OS X 10.7.2 is much faster for this ioquake3-powered game than the original 10.7 "Lion" release, but the newest Linux driver code starts out with an advantage. At the low resolutions of 800 x 600 and 1024 x 768, the Linux 3.2.1 kernel with Mesa 8.0-devel is much faster than any other configuration. However, at resolutions above 1024 x 768, the Linux performance quickly recedes. Still, the latest Linux graphics stack for this Mac Mini 2011 with the Core i5 Sandy Bridge is much faster than the stock configuration in Ubuntu 11.10, thanks to improvements within the Intel kernel DRM and Mesa 8.0. It also looks like the Sandy Bridge performance has been optimized as well by Apple with their Mac OS X driver in the Lion point releases.
When running Urban Terror, which is also based upon the ioquake3 engine, the Mac OS X performance is flat-lined across the six tested resolutions. Meanwhile, the Sandy Bridge Linux graphics performance is much faster than Mac OS X for this first person shooter.
The opposite can be said for Warsow where the Apple Mac OS X Lion driver for the latest-generation Intel hardware is over twice as fast as the open-source Linux driver. The Intel Linux team clearly has a lot of room for optimization with Mesa in this Qfusion-based game.