Does DX10 games use DX10 on 3850 and DX9 on 7series???
Yes, for the time being there are no DX10 exclusive games.
DX10 performance is low against DX9 so Can run faster on 7series??
Actually it depends on the game. DX10 settings rape some games (Company of Heroes) while they are on par performance wise in others (Bioshock). DX10 will make most games look better, but how much better is also dependent on the game. Crysis for example looks much better on very high/DX10 settings than it does on high/DX9.
In any case you can force your 3870 to run in DX9 if you don't want DX10, so it's a moot point.