The PC is due a clean...
I've done some research on your ideas. I think we've got the problem- now for the solution!
Setting vsync on ATI seems a problem. I'd be happy to do it globally, because then we could do CAS once, at which point it can be reverted. However it seems either ATI don't do this (some posters believe this) or if the "Wait for vertical refresh" slider is the same thing, it doesn't reduce fps rates in Supernatural.
I've put in Radeonpro, which is supposed to have an fps limiter on it, but that doesn't seem to work either.
The search continues...