the best comparison i could find was from Sepang.
GT500 - 1.54.306 (nismo Z)
GT300 - 2.06.669 (MRS)
FIA GT2 - 2.09.493 (997 GT3 RSR)
V8 Supercar - 2.15.849 (Ex DJR BA qualifying for Sepang 12 hour)
The v8 was being run by a privateer Malaysian team in the 12 hour race. I seriously doubt that a Top Aussie team would be able to pull the car up to the GT300 time, it would be much closer to the FIA 997 RSR's time.
if the v8's want to get serious about making faster cars get some decent aero packages for them!!