Porterfield's R4 compound is probably close to the stock GT350 pad formulation as far as effectiveness in braking. They will crumble when heated excessively while the ST43 will be unfazed.
R4 to ST43 is the only head to head racing experience I have and basically I saw no downside to the ST43. They lasted longer, took more heat, rotor life was not impacted, and the ST43 was much more effective braking. ST42, I believe, has less initial bite. Some people had problems with initial brake lock up with ST43 (no ABS!) so they went with ST42.
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R4 to ST43 is the only head to head racing experience I have and basically I saw no downside to the ST43. They lasted longer, took more heat, rotor life was not impacted, and the ST43 was much more effective braking. ST42, I believe, has less initial bite. Some people had problems with initial brake lock up with ST43 (no ABS!) so they went with ST42.
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