I have had 3 different brake setups over the years, all on supras.
Stock jspec: worked fine for general use but didn't fill me with confidence when put under heavy braking more than a couple of times. The pedal feel on these was quite spongy in my opinion.
K-sport 365mm discs 8 pot front 6 pot rear: first thing I noticed with this setup was the much grippier feel from the pedal. It's quite hard to explain without feeling yourself but it gives you a better feedback. Performance wise I would say it had slightly better stopping distances than the jspecs but the main advantage was that it would not fade like the jspecs did. This alone made it worthwhile in comparison as it was much more predictable what you were going to get from pressing the brakes hard.
Uk spec: pedal feel on these is a bit more spongy than the ksports but a lot better than the jspecs. Performance wise I would say is the same as the ksports if not better. It would not be worth upgrading from uk spec in my opinion. Only thing worth doing would be better pads and braided lines.