AMD Fusion Linux Gallium3D Performance Has Improved A Lot
Starting off with the defunct open-source Nexuiz game project, the AMD open-source graphics driver performance going from Ubuntu 13.04 in its stock condition to the completely updated stack (the Linux 3.11 kernel and manually enabling Radeon DPM and turning on the SB shader optimization back-end) has led to the performance nearly doubling in the past few months. Dynamic power management plus the use of Mesa 9.2/9.3 Git over Mesa 9.1 yielded the biggest performance boosts. While the open-source Radeon driver performance is up a lot, it's still not matching the Catalyst driver for this Fusion E-350 setup.
OpenArena also saw a good deal of performance improvements from the different upgrades made to the Ubuntu 13.04 installation, but the performance still isn't reaching that of the AMD Catalyst binary blob.
Aside from the performance coming up short, the Radeon R600 Gallium3D driver right now only supports OpenGL 3.1~3.2 and not OpenGL 4.2 as supported by Catalyst. The open-source OpenCL Radeon support also leaves a lot to be desired.