I'm surprised you are getting this Mhash/s rate in Windows. If you were able to recreate these settings in say the latest stable of Debian or the latest release Ubuntu x86_64 do you think it would be even faster?
I'm surprised you're surprised. I get 372 MHash at 930 MHz in Debian. If I could run at 1050 MHz like the other guy I should get about 420 MHash.