EDIT: The screenshots were taken at the same location in wow, without moving or anything - In Dalaran with ALOT of people around, if it matters anything.

I made some tests aswell, one with all 8 as you suggested. I had focus on the main, which then gets a max fps of 40 and the slaves 15 fps. The main would be in "01" in the second screenshot and "04" in the third. I didn't bother with "no affinity", cause that just sucks :-)

8 cores:

Second core and last core seems to work a little more than the others.

01234567:

The "main" cores is working more, but the second of the 2 cores is working craazy!

04152637:

Same as 2 pretty much!

The first solution works best, imo :-)