When upping the Xonotic image quality settings to high, the frame-rates obviously drop a great deal and the margin in relation to the Catalyst driver expands.
There you have it... While the open-source drivers continue to improve -- just not for Radeon but Nouveau and Intel too -- the performance is still a long shot from the Catalyst driver for all recent generations of Radeon graphics cards. The Radeon HD 7000 series OpenGL support with the open-source driver is not even in a working state for end-users, a half-year after the first hardware began to ship.
Additionally, it would be nice to see 2D color tiling and PCI Express 2.0 enabled by default for recent Radeon hardware. The 2D color tiling will likely be picked up by default (it already has been for R300 through R500 class GPUs) in the coming months while the PCI Express 2.0 state remains unknown. I have yet to run into any issues setting radeon.pcie_gen2=1 and the benefits are quite clear. I also have not seen any Phoronix readers in past months report any PCI-E 2.0 problems.
If there are any outstanding PCI-E 2.0 issues, they should be addressed rather than blanket disabling PCI-E 2.0 for all -- especially with this being an item that needs to be set at kernel boot-time and not something that Linux desktop users widely know about or is as easy as clicking a checkbox from a GUI.