Thanks for the reply. I remembered how to do it, but didn't know if it still needed to be done.

Anyway, I went ahead and enabled it (specifically, I did "14" so the first core would remain available for the OS). Not only did my occasional slow-downs on the main toon stop (FPS would dip below 12 for several seconds with no good reason), but the CPU usage for each copy of WoW dropped 50% to 75%. Much better than I remembered; so much better in fact that I may even consider getting back up to 5-boxing.