Vex 393 motor with 12V supply
I'm inclined to agree with you. A suggestion: Find out which the nominal current (or the maximum for the load you want to move) when operating at 7.2 V DC. Let's call it "Im". Then apply a 12 V PWM signal  varying its duty cycle from 0 up to reach the "Im" current. Done. Now just make sure that the applied duty cycle is not more than that.