Ubuntu 15.04 Offers Faster OpenGL For AMD Radeon GPUs On Open-Source
The Warsow gaming performance was only boosted for the newer AMD graphics cards with the R600g driver in a relatively mature state.
Xonotic saw performance improvements with the RadeonSI driver as well as the R600g-powered Radeon HD 6870. The lower-end Radeon HD 6000 series hardware didn't see any performance change when upgrading from Ubuntu 14.10 to Ubuntu 15.04.