Sry, I was a bit quick on that one. After setting VDDC in Atitool it will stick when lauching a 3D app (at least 3DMark). It will not stick if you try to alter the core-clock via Atitool.

So the way to go is to set VDDC in Atitool and clock with another software.
Or, set coreclock first in Atitoll and then alter VDDC.

btw, I did try Powerstrip, somtimes it worked, somtimes it didn't.