I got to run my 21.5 M3 in USGT....it ran a lot cooler and seem more torque out of the corners.
my new 17.5 M3 was tested at our track by a very respectable driver in 17.5 stock touring car, and he thought it was a good motor.
I plan on running the 17.5 SS version in my TC6.2 on our next club race....Ill report back with my info