With the APITest OpenGL 4.x tests developed by John McDonald at NVIDIA who is now working for Valve on their Linux-related endeavors, the AMD Catalyst driver gets absolutely annihilated for these GL4 micro-benchmarks.
It's been brought up by other game developers how the APITest performance shows the proprietary NVIDIA Linux graphics driver doing much better than the AMD Catalyst driver. With my own testing of three recent GPUs on each side, this is indeed the case.
The NVIDIA graphics cards I tested were the GeForce GTX 680, GTX 750 Ti, and GTX 780 Ti. The AMD Catalyst cards tested were the Radeon R7 260X, R9 270X, and R9 290. In nearly all of the core OpenGL APITest cases, all three graphics cards were killing the AMD Southern Islands hardware on Catalyst.
All testing was done from an Intel Core i7 4770K system running Ubuntu 14.04 64-bit with the Linux 3.13 kernel. The latest proprietary drivers were used: NVIDIA 337.25 and the Catalyst 14.6 Beta (fglrx 14.20.7 / OpenGL 4.3.12967). The APITest benchmark was driven automatically in a standardized and reproducible way using the open-source Phoronix Test Suite.