The results from OpenArena immediately illustrate results that are vastly different from what we experienced with Compiz 0.8 and the drivers at that time. With the three ATI Radeon graphics cards spanning different hardware generations, running Compiz actually led to an increase in frame-rate. Though based upon the X1950PRO and HD 5450 being close to 60 FPS, it's possible that it was trying to v-sync in the non-composited environment, although vblank_mode=0 is forced by the Phoronix Test Suite. The Radeon HD 5450 with Metacity was at 22 FPS. With Compiz running on the R300g-powered X1950PRO the frame-rate jumped to 105 FPS and with the R600g-driven HD 4670 it raised up to 88 FPS.
The X1950PRO couldn't be tested with the Catalyst driver since the R300 through R500 ASIC support was dropped back in 2009, but with the Radeon HD 4670 and Radeon HD 5450, the frame-rate was the same regardless of a composited window manager or not.
With the two NVIDIA graphics cards using the Nouveau Gallium3D driver, there is a performance drop. It is a measurable drop in OpenArena with the frame-rate going down by nearly 40% for the 8600GT and almost 50% for the GeForce GT 220.
When using NVIDIA's proprietary driver, there was also a performance drop. At least with NVIDIA's proprietary driver, the cost of running Compiz was less detrimental compared to the Nouveau Gallium3D driver.