dx10 isnt just graphics. dx10 and 11 introduce efficiency savings elsewhere as well. If they are able to just assume everyone has access to those it should yield something new and/or uncap the extremes in the game maybe including player numbers
Ive yet to see a dx10 game which did that though, long time waiting for nothing much so far thanks to consoles but mainly the failure of vista adoption pretty much, this rumour is possibly true just maybe because win7 isnt crap and has sold and is selling far more widely
crysis 2 developers claim that with these optimisations they can get twice the fps compared to crysis with the same graphics quality. lets wait and see