Just my experience with 4x4 SC's we ran 6.5 540 and they ran really hot and lacked enough torque. We jumped up to an 8.5 and had good success. We eventually went to a 7.5 and shortly after installed a 13mm rotor and found a great combination. We have always run the Tekin RS Pro. Though I must say that with 550's the Tekin ran a bit hot.