I'm not sure if this has been discussed (I'm not keen on digging through 38 pages of comments) and I've not found anything through the search function, but I've found that if I use older drivers for my radeon HD 5750, specifically the 12.8 drivers, I get a 50% or better increase in performance. That is, i go from barely reach 30 fps on low settings at 1600x900, to almost constant 50 fps on frozen city (my worst performing map), and an almost constant 30 fps on river city on high settings.
Now that's still not great, but I'm running a 3 year old system, i7920 & radeon 5750 as mentioned with 6gb of ram, but the difference means the game is fun, playable and looks great, vs frustrating, slow and plain looking. So i'd recommend everyone using ATI/AMD video to consider rolling back to older drivers, and those with NVIDIA hardware perhaps looking to making a similar change themselves.
Newer isn't always better!
Edited by Niko Snow, 16 April 2013 - 05:05 PM.