VDrift tends to perform well on the open-source drivers relative to the closed-source (NVIDIA and AMD) drivers. With the latest code when enabling these "tweaks", it is still the case.
In recent times, the R600g driver has begun to perform much better in Lightsmark, the OpenGL 2.1 lighting benchmark. The latest driver code increases the Lightsmark frame-rate for the R600g driver by 2.76x or by 4.06x when tweaking the driver.
These results for comparison purposes are available on OpenBenchmarking.org. As you can see from the results, there are many great things happening in the open-source driver world for Linux. This discussion will surely continue in the Phoronix Forums.