Here is what I got from just using MoI's counter: (the speeds are guessed averages)

Theads: Speed ms:

1 163 (4 cores active)
2 145 (6 cores active) What?
3 153 (7 cores active)
4 160 (8 cores active)
5 160 (8 cores active)
6 160 (8 cores active)
7 160 (8 cores active)
8 160 (8 cores active)

So Michael, it seems that the 2 thread setting is the way to go for now.
Now, I might try to figure out how to get Fraps to run a controlled timed test, but I hope this data is slightly useful.