This from the FAQ in Avid's sales guide (issued at NAB this year):

I've played with Media Composer Adrenaline on both Mac and Windows and there is a real performance difference. Are you making the Mac version run slower?
Of course not, we optimise for both platforms. At this time we are seeing significantly higher real-time performance on current Windows-based workstations than G5 workstations from Apple.

So now we know!