First of all, I've been in/raced the car you posted a link to when it had different turbos, and it was RIDICULOUSLY fast with several hundred LESS HP. It made my Stage 3 TT look like it dropped anchor. I agree with everyone else, a TTZ with even a middle lever turbo upgrade is all the power you need, you should really experience it before you want more.
Second of all, if you want to "beat Supras", there are street Supras running 1200+ rwhp (I think at the TX Supra event someone made 1400rwhp?). An RB motor from a GSt Skyline ain't gonna cut it without so much money into it, you may as well have just bought a Supra.