With the rendering capabilities of today's graphics cards I doubt that a 10 to 15% drop will matter much in terms of real world game playability. We often see (and sometimes base our buying decisions) on frame rates and to a degree that's the way it should be for what has high frame rates in a game now, means future games should be quite playable also therefore making our purchases last that little bit longer.
Where frame rates are concerned, we can't tell the difference beyond a certain level. When Vista comes to launch a new breed of GPU's will undoubtedly be unleashed upon us, all of which will probably cope eyewateringly well with this reported performance drop, true or not.
My opinion anyway ;-)