It won't hurt the motor at all. Your ESC, and batteries plus the motor will run cooler. Thing is will it be enough to pull the Mustang along well or are you going to be at or near full throttle to have it fly well. You still have room to play with as far as prop diameter and pitch the motor is rated for 55 amps continues.
Just curious what is the total AUP wgt. of the mustang?