TOTB1...

Two 3 door rear wheel drive RS's:

Andy Wornell 126mph at 500m
Rob Wiles 123mph at 500m
Both with claimed 500+bhp on the website.

GTRs at 500m:
Andy Barnes 130mph (550bhp est at the time)
Glen H 127mph (540+bhp back then)
Mike Smith 126mph (515bhp back then)

The GTRs also pulled higher 1.25mile times, so in conclusion:

The GTRs were better off the line.
The GTRs were travelling faster at 500m (ie were not being caught)
The GTRs were travelling much faster at 1.25miles.

I use the speed at 500m, since that is a determinant of rate of acceleration from meaningful speeds ie 30mph+. The traction really only has an effect on the ET (elapsed time), hence why I ignored the times, but they were also massivley faster.