While the open-source Radeon and Nouveau Gallium3D drivers have a limited level of OpenCL support via Gallium3D's "Clover" state tracker, it's not too useful. Radeon Gallium3D on OpenCL can run some simple demos and even a bit of open-source BitCoin mining, but it's not enough to be useful yet or really performant. There isn't any tier-one Linux distribution shipping this open-source OpenCL support yet by default and it will likely be some months before it's really useful for end-users.
Anyhow, on the proprietary AMD Catalyst and NVIDIA Linux drivers I ran some new OpenCL benchmarks via LuxMark on the Phoronix Test Suite for several different GPUs. Tested hardware on the NVIDIA side included the GeForce 9500GT, GTX 460, GTX 550 Ti, and GTX 680. Meanwhile, on the AMD Radeon side was the HD 5830, HD 6450, HD 6770, HD 6870, HD 6950, HD 7850, and HD 7950. The Catalyst 13.6 Beta (fglrx 13.10.10 / OpenGL 4.2.12337) was controlling the AMD Radeon hardware while the NVIDIA driver was the 319.32 release.
These results are mainly being put out here as reference numbers for those wanting to do comparisons -- simply installing the Phoronix Test Suite and then executing phoronix-test-suite benchmark 1306304-SO-FEDORALIN07. However, a bit surprising were the AMD Radeon GPU numbers.
NVIDIA cares more about CUDA, but with the latest drivers running, the AMD Catalyst performance on the range of hardware tested was extremely good. The high-end Radeon HD 6000 GPUs were even faster than a NVIDIA GeForce GTX 680 Kepler and the HD 7000 series were multiple times faster. These OpenCL tests were running on just the GPUs for each driver and not the CPU.
Find the rest of these OpenCL NVIDIA vs. AMD details from Fedora 19 Linux in its OpenBenchmarking.org entry. Compare your system's OpenCL performance against these GeForce/Radeon GPUs by simply running phoronix-test-suite benchmark 1306304-SO-FEDORALIN07 to carry out the test install and execution process in a fully-automated and streamlined manner.