JNS,
Here's one data point for you.
Last December, my GTS dyno'd at the same shop at 456 rwhp and 490 ft-lbs. The only mod I have since then is the 3.55 rear end, and Saturday I dyno'd at 448 rwhp and 480 ft-lbs. So, for me, assuming good SAE correction factors, consistency of Darren's dyno...