If FDR is restricted, its even better to have a few different rotors that you can fine tune to the track you're racing on. Smaller rotors for top end, bigger rotors for good infield performance.
Here in Malaysia, we run only 11.5T motors and no modification to the motor is allowed for spec one make motor races.
I thought that 14mm was a little too big as well but for the heck of it I tried anyways and found that it worked great. However, you'll have to use one of those esc's that has high software timing too get back the top end.