If you symlink your WoW data folders my impression is that with a fast HD (2x Raptors in RAID0 in my case) you don't spend much time waiting on the HD. The limiting factor on perf for me seems to be CPU load in areas with lots of other toons: Dalaran, Wintergrasp, etc.

Have you tried using perfmon or another tracing tool to verify that disk access is really limiting your perf?