When using VDrift in the past few months with the latest open-source drivers, the performance has been rather interesting. The Mesa and Gallium3D drivers tend to do much better than their respective binary drivers do and/or the delta between the two is much closer in cases where the open-source drivers fall behind. This has only happened in the past few months, and no, it is not because this racing game is falling back to not using GLSL shaders or anything on the open-source drivers compared to the AMD and NVIDIA binary blobs.
The R600g Gallium3D driver was broken with VDrift at this stage in the Mesa 7.11-devel cycle, but the Core i5 2500K performance on its classic Mesa DRI driver was right alongside the Radeon X1800XT on its Gallium3D driver and a few frames ahead of that was the Radeon HD 4830 on the Catalyst driver. The Core i5 2500K pulled ahead of the Radeon HD 4650 on Catalyst, GeForce GT 220 on Nouveau, Radeon HD 4550 on Catalyst, and Radeon HD 5450 on Catalyst. Yes, the Intel Mesa driver really is pulling in front of the Catalyst driver in these cases, but again VDrift is sort of on its own planet in terms of unique performance for the different drivers.