Ran the GTB 4.5R again over the past weekend. Changed gear ratio to 26/102T and made final gear ratio to ~1:8.83, still running 5cell IB4200 pack. After a few run, the car stop on the track but I can still hear the motor rotating ... went to pit and found the rotor broke.
Change to the latest Novak 13mm rotor (more torque according to the manual) and adjust the gear to 27/102T (final gear ratio 1:8.5). Acceleration is great altho the top speed is not as good. I felt that I can be more agressive on the gear ratio as the motor/ESC is just warm after run.