ok i looked it up couldnt find 2004 resutls fast enough they might follow later...but considering the 911's are in a class lower, look at how few laps there have been between the 1st vette and the 1st 911 in the last couple of years (and in 2001 a couple of 911s actually beat all corvettes!):
2005 -17 laps
2004 ?? laps
2003 -6 laps
2002 -13 laps
2001 +5 laps
2000 -8 laps
at the +/- 330-350 laps they cover, and considering the 911 is a class lower, most of these differences are nothing at all...so dont be too sure that porsches wouldnt beat the vettes if they were in GT1...and otherwise porsche could turn the GT2 roadcar into a racecar for the GT1 series if thats allowed, and beat the crap out of the vettes
also these porsches were all privateer teams, while the vettes were sort of factory teams (pratt and miller being a sort of "corvette racing" just like prodrive was aston martin racing in 2005)