This is the sour conclusion the testers from NeoSeeker drew when they tested World in Conflict (a Real Time Strategy game) stratégie temps réél ou RTS) to compare the results in both Microsoft graphics modes.
The conclusion is big: the difference is not, except that performance is more or less divided by two when you run in DirectX 10. The game forcing you to move to DirectX 10 (remember that this is also forcing you to switch to Windows Vista) is not yet there.