I oc-ed my 2500k after not getting decent performance at stock with two overclocked 6970s. Once I hit 4.3ghz it stopped dipping under 60fps almost altogether with mostly ultra settings (but with 2xMSAA), even on the largest maps. I certainly noticed quite a difference. Interestingly, for me the perfoverlay didn't show the cpu as an issue, but when it was a bottleneck it became a rough jagged line (as did the graphics card line), once overclocked this stopped happening. I think this and the fact that all the tech sites focussed on the singleplayer for benchmarks and proclaimed that the cpu made no difference has had a lot of people assuming the performance issues are all graphics card related.
Unfortunately I can't get my cpu any higher without ridiculous voltages but it would probably help to go even further. So yes, night and day difference at a higher oc for me.
sorry for the late response,i was busy somewhere else.