Interesting piece over on Geeks3D - it seems that the latest Catalyst 8.8 drivers released a few days ago by AMD have a built-in feature that detects when the user has the FurMark OpenGL benchmark running and downclocks the GPU on the Radeon HD 4850 and Radeon HD 4870 to keep the temperature down.
How can you spot this? Simple, just rename FurMark.exe and watch the performance change!
Even if you don't have FurMark installed you can see this for yourself. Here etqw.exe from Quake Wars: Enemy Territory has been renamed to FurMark.exe and there's a huge drop in performance noticed.
There's no doubt that FurMark is a pretty brutal benchmark, especially when run in Xtreme Burning mode but I can't understand the inclusion of this undocumented downclock.
I've approached AMD for comment on this issue.