Indeed, in the ALMS the C6R's seem to be pretty dominant at the moment, in the FIA GT the Maserati MC12's are dominat with the Aston Martins coming in second in the overall standings and the C6's coming in 6th so far this season, but I have to note the Vette's in the FIA cup arn't factory backed. I think what series the car's race in will always provide different results, over here it may simply be that Aston Martin and ProDrive have put more effort into creating the winning team for the home crowd than for the ALMS. But in reference to performance, the DBR9's are capable of what th C6R's are.
Regarding car's in higher classes only being marginally faster than GT1 cars, this happens at some tracks and in certain weather conditions, P1 or P2 car's are faster, but not outragousely faster like a lot of people seem to think. So if the track is a track where a GT1 car can reach more of it's potential than a P1 or P2 car, the difference will naturally be lessen.