Basically, radeon is faster than nouveau except for drawing circles.
Phoronix: Nouveau vs. Radeon 2D Graphics Performance
Earlier today were the results from a 9-Way Low-End NVIDIA/AMD GPU Comparison On Open-Source Drivers using the open-source Radeon and Nouveau Gallium3D drivers. For those more concerned about the 2D Linux desktop performance, here are some results for reference...
http://www.phoronix.com/vr.php?view=MTMxNDA
Basically, radeon is faster than nouveau except for drawing circles.
Perhaps in this three-dimensional land. However, this was a 2D graphics test. And, as everyone knows, circles are the smartest beings of Flatland. So I guess Nouveau is working smartly.
Seriously, though, Radeon's official AMD support has been paying off drastically lately, hasn't it? Especially for evergreen and older hardware.