[Mike Hudson]

is the rotor in the motor bonded or sintered?
I run currently run a 5.5L sintered rotor in my X6 (conversion of a B4) with a 20t pinion/81t spur i'd play around with 20/21/22t pinion on a 81t spur with a 6.5 sensored motor, just check the temperature of the motor after each run and if you can't touch it with your finger for a fair few seconds without it burning you then it's geared too high so drop down a tooth or two if needed, hope this helps
Mike.