I was at the Eastern Creek World Time Attack, there was the High Octane R34 GTR that did a lap time of 1:34.1090 and a Mal Rose Racing Holden Commodore with a lap time of 1:39.3230.
Fair to say both cars were in the same pro class with a 5secs difference, so I would naturally think a modified R34 GTR is a faster car around a track than a modifed V8 Holden.
I have not mentioned the CyberEvo present at the event took the 1st place with a lap time of 1:30.5870, could potentially done a 1:29 but did not due to near blown engine which they tried to fix overnight nonetheless still managed to win the pro class event.
Not sure if you guys know, but the CyberEvo does laps around Tsukuba track in Japan in the 54secs range similar to that of the MSpeed R34 GTR, so we can pretty much use the CyberEvo lap times around Eastern Creek and compare that to the Mal Rose Holden Commodore, meaning a near 9 seconds difference.
Stock R34 GTR vs V8 Supercar, thats not even fair to even compare.
Modified R34 GTR vs Modified V8 Holden, as above R34 GTR clearly is faster.
GT500 R34 GTR vs V8 Supercar, I wouldn't have a clue but my money is on the GT500 R34 GTR