Well I have driven eastern creek with full trim, subbie in the boot etc then I took the subbie, spare, jack, carpets and sundry other crap out and did some more laps. The back end was noticeably looser in braking situations both straight line and corners but it was harder to tell whether it made any difference with the power down. Perhaps that's because I don't have all that much power though.
This is with stock GTT suspension. I guess if your suspension was adjustable you could compensate.