Cool comparison, but a little unfair given the MM is technically a 4s/100A ESC while the MGM is a 6s/160A ESC.
About the overvoltage thing on the MM; when braking the motor can send some pretty high back-EMF pulses into the circuit, then to the battery. This is probably what you are seeing. It's apparent that 5s is kinda pushing it IMO. Maybe back off on the brake force, or use mechanical brakes to help with that.