These latest results have Darktable 2.2-rc1 tested on some more Linux systems around the office, all with Intel CPUs. I had attempted to run some HD/Iris Graphics OpenCL tests for Darktable using Beignet, but wasn't able to get it working on any of the systems, thus the CPUs listed on these results are CPU-only data points.
With the exception of the Intel Core i7 6800K, the GPUs were faster than the range of processors tested for this comparison.
Or in another scene, the CPU results were slightly more competitive with that workload not scaling as well on the GPUs.
See this morning's Darktable article for more background information on the testing and the photos used. Some have also requested some OpenCL benchmarks with GIMP, but unfortunately I am unaware of any GIMP equivalent to Darktable's darktable-cli for being able to fully-automate the benchmarking process in a repeatable manner. If any Phoronix Premium readers have any other test requests, feel free to let me know.
If you have Darktable 2.2 currently installed on your Linux system with OpenCL working, using the Phoronix Test Suite you can compare your own system's performance by simply running phoronix-test-suite benchmark 1611284-TA-DARKTABLE72. Or go to the OpenBenchmarking.org page if you want to learn more about these systems under test.