With all this talk of brakes, i thought i'd throw my two pennies in.
I've got J-spec Brakes, with standard pads, but i had a track day coming up.... Couldnt afford the upgrade so just uprated the pads to Porterfield Full Race pads, fitted them and noticed improvements, went on the track day must have done about 20-30 laps, no brake fade, and excellent stopping power, however i didnt realise how good they were until i took them out again and the standard pads were so shiteeee i almost ran in the back of people, so i dont think its so much the discs but the terrible pads that are put with them.
Never been in a UK setup so cant compare, have been in a Kad setup with UK rears and they are very impressive...... But just wanted to say that i think the pads are the culprit more than the discs themselves when it comes to J-Spec.