I had J-specs on a standard Supra - no mods at all. Even then the brakes were bad. Then I went BPU and they were very bad. Then I upgrade to Brembo fronts and J-spec rear, not too bad. Then I went from Auto/Tiptronic to Manual, and now they feel inferior as the amount of reaction time has changed as the gears are shorter.
If you had a BPU and had the gas flat on the floor, you'd be accelerating at a certain speed. If you were in a single, you'd be accelerating maybe twice as fast (or at least feel like it), and by the time you realise you need to brake, and have started pressing on the brake pedal, you'd have covered twice the distance as in BPU, so therefore would need better brakes. If you were both going steadily at 50, then the turbo obviously makes no difference, but you have to take into account the potential requirements for braking, and the bigger singles will be doing from 50 - 80 faster than you can imagine, and suddenly going from accelerating to braking at that sort of load needs better brakes.