Confirm it - no, not with certainty.
However:
I could reach comparable speeds on the same machine with linux, but only with very extreme values for -f (poclbm), which would make the UI completely unusable. Also, I have two jobs for each GPU, one primary with small -f and a secondary with larger -f, which works like a charm on windows (if primary pool goes down, secondary will take over). Under linux, this did not work well - both jobs would be executed at similar rates.

On windows, everything is fine.

BTW: I was able to fix the problem on the Hyper-V, where the second card was not detected - seems it depends which port you plug the dummy in!
Also: LogMeIn is damn sweet!