Here's an interesting finding: at least when running under Wine, Half-Life 2 is faster on the open-source AMD Radeon Linux driver than when running on the proprietary Catalyst driver. There are also some other similar results where these Windows games have the advantage when running on the Gallium3D open-source driver.
Stefan Doesinger, a developer at CodeWeavers and someone that's long been using the Phoronix Test Suite and OpenBenchmarking.org for conducting Wine performance tests and running benchmarks on a near-commit basis to watch out for Wine performance regressions, shared some interesting numbers.
What we have in this article is a set of Wine performance tests using the Radeon Gallium3D driver for a number of different Wine revisions and then at the very end (right side) using the proprietary Catalyst driver. Tested was a Radeon HD 5700 "Juniper" series graphics card on fglrx 8.96.4 and then on the open-source side was the stable Mesa and Linux 3.2 kernel from Gentoo while running atop a GNOME2 desktop. The CPU was an Intel Core i7 860.
All of this testing under Wine is using the very recent Wine 1.5 development code.