Phoronix published a review on the Radeon Open-Source Performance Over Three Years, Compared To Catalyst A quote from the article:

Yesterday on Phoronix I published results showing the Radeon Gallium3D performance getting close to Catalyst for the Radeon HD 5000 series and newer GPUs that are supported by the mainline Catalyst driver. On the open-source side the hardware is supported by the R600 and RadeonSI (for the GCN hardware) Gallium3D drivers. Yesterday's results showed that on Ubuntu 14.04 for many OpenGL workloads the R600/RadeonSI Gallium3D drivers were now ~80% the speed of the proprietary Catalyst driver in many instances. While this is a great feat, how does it stand for older generations of AMD Radeon hardware? In this article are Radeon HD 4870 benchmarks looking at the performance of the open-source AMD Linux driver over three years of Ubuntu Linux releases and compared to the legacy Catalyst driver from 2011.

 Radeon Open-Source Performance Over Three Years, Compared To Catalyst @ Phoronix