FWIW, I went from a 2500k to a 2600K and it made a BIG difference. (stock speeds on both)
But then again, I 5-box at 2560x1600.
Maybe it makes a bigger difference at higher resolutions.
With the 2500K, I was pegging all 4 cores and it would get choppy in Stormwind and 40 man BGs at times.
With the 2600K, I'm at about 50-60% on all 8 "cores" and it never gets choppy anywhere.
Was definitely worth the extra $100 for me.

Other relevant specs:
8GB RAM
Nvidia GTX 580
64GB SSD for WoW.