Our track runs GT3 with the silver cans and the guys gear the 04's to the moon and back. No one is burning those things up. They some serious sized pinions on those cars. I think they run like a 42 or 43 tooth pinion and like the Tamiya 120 spur gear.
I want to try my 03R in that class but I think it is better suited to the GT2 class with stock motors where I can change brushes and springs to suit my car. It will be very hard to keep up with 04's in the GT3 class using those silver can motors unless you have an old silver motor that is tweakable and that is not TCS legal to my knowledge.
I would like to cut down my gears some like that Integy gear set from ahile back. The main diff gear is very narrow compared to the stock wide ones. Really cut down the friction on the car.