The Maserati MC12 team Goh tested was 2-3 sec slower than the top GT500 on test days. But they hadn't changed a thing at all with the earodynamics. So I guess it could be possible while having a great team of engineers getting the cars earodynamics changed within the GT500 regulations.
I think it has also something to do (well in combination with the rest) that the rear diffuser (which is placed more to the middle of the car on most GT500 cars. More between the rear axle in stead of behind it as with GT1 cars) This, I think will keep the car steady when it can be driven faster through the corners (more to the rear will handicap this through the corners)(But this is just a noobs guess after some observation
) (I think with earodynamics the 550 and DBR9 could be compared best to the old Supra GT500)
Hope it just races. Wonder how it will look with asian sponsors