I'm a little late. I'm not a big fan of the Porterfield R4S. I didn't like the feel on the street or track. Yes, they work but they do neither really well. On the street, the feel was mushy IMO. At the track they were OK until I had to brake from high speed (125 to 40) then they overheated and melted. I'm a Carbotech fanboy. I loved the XP8, still have them on the rear. Moved up to the XP10 and was in love again. The make a lot of noise on the street but the pedal feel is great. Now I have XP12 on the front and I found another love. Incredible. They are expensive, they don't last forever, they don't eat the rotors, they dust like crazy, and they stop like no other pad I've ever used. For autocross, any pad will do. If your braking from high speeds constantly do yourself a favor and try at least the XP8's. Perhaps the Bobcats? Never tried them myself but based on the other Carbotech's I feel comfortable recommending you try them.

Les