The main thing you have to worry about at a circuit day is cooling, namely engine and brakes. For a heavy GTR, I'd say brakes are quite up there on the list of priorities, both do depend on how hard you're driving too. All the rest should be fine
And mod for mod, a GTS-t will always handle better than a GTR, especially through twisty stuff like turns 7-9 at Mallala. ~250kg lighter overall and much better weight distribution means both turn in and mid corner are greatly improved, but corner exit is where the GTR has the advantage. For the lower power cars, getting power down isn't much of a problem, so the GTR doesn't really have a gain here.
Garn, let the GTS-ts out Promise a few of them have motors to put RB26s to shame