Likely Radeon Gallium3D Regression On Linux 3.14 + Mesa 10.2
The HD 6770 didn't see any regression when running Unigine Heaven. For the older Radeon HD 4000 series hardware, with Linux 3.14 and Mesa 10.2-devel at least they can now handle running Valley and Heaven with their OpenGL 3.3 compliance.
Ditto.
The Xonotic first person shooter saw lower frame-rates with the newer code.