The performance is still scaling linearly up to 3.2GHz. Linear scaling alongside CPU or GPU means you don't have a fast enough component. :lol Going from 3.2 to 3.7GHz results in half the increase, so as you get towards 4GHz, improvements to CPU speed seem to pay back less than before.
Keep in mind as well, a X4 GHz is inferior to a i7 GHz, as a 2.6GHz i7 comfortably beats a 3.5GHz X4 in Tom's benches.
What's wrong with the ~60fps @ 2.3GHz with a 8-threaded 4-core i7? Well, nothing until you have double the units on the screen as they did in that test! Then you're likely down closer to 30 fps. They only mention they tested a 5-minute 4 player battle, so it's hard to say exactly how stressful the test really was.
I believe the original question was what was necessary for maxing SC2, and I was just pointing out that CPU makes a hell of a difference in that game.
Look here, and you'll see at lower resolutions (1080p and below), you're generally CPU bound, as a $150 GPU (5770) scores roughly the same as a $600 one (XFire 5850), while that is not true of the CPU.