Comment 104 for bug 96991

"radeon.modeset=1" needs to be added at the end of the kernel line, 1 space after "splash". [Well, that's how it works for me :) ]
But do note , there is regression with using KMS , the FPS will be much lower than without the KMS. [in my case nearly half]
So the system might *seem* a bit sluggish, well it is actually the slow rendering and not the cpu slowness.

For any other info > https://wiki.ubuntu.com/X or you can ask for help in irc #ubuntu-x .