I love that Supra, the car is an absolute giant killer and a blast to drive. We're planning on a trip down to NC sometime soon, my other buddy has a 96 Viper and we've been talking about lining those two up for ages. The guys in my friends club are running some insane numbers. I'd say the average is about 500 horse at the wheels, on basically the stock bottom end. Thats the great thing about that Toyota motor, it's so friggin over-engineered that it can take massive power increases and still keep right on tickin. Most of those guys running 500 horse have been doing so reliably for thousands of miles.
Some of the really serious guys in his club are putting down the crazy numbers as you've mentioned. I've got a vid of a guy from long island throwing down 820 at the wheels, I thought that car was gonna break the straps. Most of those cars have replaced the twin turbo setup with a big whopper single turbo. You loose the low end responsiveness and get a whole mess of lag but when that 30 or 32 lbs of boost comes on you better hold on to your a$$.
Problem with the 300ZX was fuel delivery. Unlike the Supra, whose fuel system could probably keep up until somewhere around 450 horse, the Z's system gets maxed out much quicker and you need to go to bigger injectors at much lower power levels. Also, from what I've been told that motor also runs out of breath at a much lower rpm than the Supra. Still a way nice car though with lots of power building potential. However I think you can get more power for less money out of a Supra.