GT1 cars are easily on pace with GT500 cars given the right team and proper tires. The MC12 here is adapted to steel brakes and has more weight but still is only 2 seconds back.
Why GT500 cars are in Group 2 but the DBR9 is in Group 3 is a mind boggling decision. Especially when before 2010 GT500 cars did not go over 500 HP.
Comparing a Maserati MC12 GT1 (which is still 3 years off-pace from the top of GT500) with the Aston Martin DBR9 is like comparing a steamtrain to a space shuttle. Simple fact is the Maserati MC12 was completely dominant in GT1 and practically single-handedly killed GT1, whereas the DBR9 was miles behind in performance, and by the end of each race, quite literally.
It would make infinately more sense to put the DBR9 in Gr.3, where it actually can compete, and gets the same lap times round Silverstone than a modern GT3, instead of having it sit utterly useless up against much faster Gr.2 cars.
In reguards to the MC12:
If you think 3-4 seconds slower per-lap is "on pace", then you have me speechless.
Let alone in 2009 Super-GT, in every GT500 race the Aston competed in, it finished around 8 laps behind the GT500 winner on avarage.
(despite weight-handicap / BoP per race)
We now have an option of two Astons for Gr.3 both tailored towards different driving styles, one with less grip but more brute force in power, while the other being more nimble and plucky.
What we have now, is choice. And choice is good.
Especially in racing, of which choice fits your style best.