The motor actually gets full battery voltage at all times - the ESC simply sends it in high-frequency on-off pulses - and it widens those “on” pulses as you give it more throttle until eventually you hit full throttle and the pulse is just fully on with no off.

Also - What gigaplex said is correct. Adding mass to the flywheel or any part of the rotating drivetrain will only show up as a tq/hp loss when using an inertia-type dyno that only calculates torque by measuring the rate of acceleration instead of measuring the actual torque like a brake dyno does.