Due to a change found in the Linux 3.12 kernel, the AMD Radeon GPU performance is a lot faster with the open-source driver for certain configurations and the Nouveau driver performance is also faster depending upon the CPU and GPU. Now the latest in comparing the Linux 3.11 and 3.12 kernel performance, I can confirm some small performance changes with the binary Catalyst driver.
Due to the CPUfreq changes within Linux 3.12, depending upon the GPU, CPU, governor, and OpenGL workload, the performance can be improved compared to Linux 3.11 and prior. For the quick Catalyst testing done this morning, the Intel Core i7 4770K Haswell system was again used and an AMD Radeon HD 7950 graphics card benchmarked with the Catalyst 13.11 Beta for Linux x86_64 (OpenGL 4.3.12458 / fglrx 13.20.6). Fortunately, AMD's Catalyst already works with Linux 3.12.
With the Radeon HD 7950 the performance changes from Linux 3.12 didn't tend to be as dramatic as with the open-source Radeon DRM/KMS driver paired with R600 Gallium3D, but still noticeable for some benchmarks when monitored using the Phoronix Test Suite. Be sure to check out the earlier Linux 3.12 kernel performance articles linked to from above if you haven't already.