I have them on my track car now for 3 years and on my last supra (Same kit) for 1 year and they have been fine so far.
As with anything braking is down to pad choice, fluid and tyres not just bigger discs and bigger calipers.
UK spec brakes have been proven on the track but IMO I found the feel awful.
I am looking to change my brakes from Ksport to AP Racing when I have the chance but this is mainly down to the fact that I may need to get hold of spare parts quickly and in my experience the lesser known brands usually have longer spares lead times, although with the Ksport kit I have yet to test that (they have not let me down at all).
I would say whacking on 8 pot on the fronts is going to bugger your brake balance, you may want to put UK rears on or at least at least upgrade the rears with one of the Ksport rear kits. No point making your front tyres do ALL the braking work, not only will you actually lose performance it could cause a nasty accident.