So the PEAK was 315, average something like 280 W, and this in a single benchmark run. Hence the card falls to the under 300 W in games.

My point is that there won't be a card which just uses more than 300 W constantly in games. Most probably there won't be a retail card exceeding Mars' power consumption. And as PCI-E standards should be backwards and forwards compatible, the 300 W standard limit isn't going anywhere anytime soon.

So, all in all, this means that GF100 X2 card won't be much more power hog than the current X2 cards.