No,
I spent since launch running the game on an 8 core 2.8 xeon (heaps of ram, ssd, 560ti, win 7 x64, aero off, xp3 compatibility checked, etc, etc, etc). Always with cpu and gpu usage meters up on an extra screen. The game never utilized all 8 cores.
Typically in large illum fights my fps would be 4-8 fps with the utilization meters sitting <25% for both cpu and gpu. Never in those instances would one individual core be maxed.
I just put together an i5 with the 560ti and with barely a bump in clock speed and 4 less cores it runs the game much better.
The game mutithreads sure, but it certainly wasn't effectively utilizing more than a couple cores.