I thought I'd share these tweaks to improve 3D game performance for RadeonHD cards. Maybe it's of some use to others.
Option "ColorTiling" "True" # not sure if this does anything
Option "ColorTiling2D" "True" # this is currently turned off by default
Option "SwapBuffersWait" "False" # this might cause some tearing though I don't see any
Add the boot parameter radeon.pcie_gen2=1
Start your game with
For me it changes the frame rate from well below 60 fps to well above 60 fps in Nexuiz.