Also using Carbotech (but US) since years. Best pads for track after getting disappointed by all the big brands. They always works fine. When a big brake is fitted to the front i would use XP12 on the front and a XP16-20 on the OEM big rear.
Ive got carbotechs on my turbo. They are great pads but they are bloody noisy and also very dusty.
Ive heard porterfields are just as good and doesnt have the negatives above. I will be trying them when my carbotechs are due for replacement.
Porterfield R4's are probably as good as carbotec XP8, R4S certainly aren't good enough. I chewed through most of a set of R4S doing one day at bedford and they did fade where the XP8's didn't. R4S is a perfectly fine street compound though, where the xp8 tends to need heat going through it for it to be fully effective.
The R4 compound is not recomended for road use however, wheras the XP8 is ok for occasional use on road.
If you don't mind swapping around pads a set of R4 and R4S would be pretty good. I can't be ar$ed so I just use the carbotechs.