eddy, do you actually know whay a Kj is?
a Kj is watts per second.

If a disc has 4 Kj potential energy, and a 4000 watt spinning it, if it takes 1/4 second to stop it, you have applied 4x4 kj kinetic, and 4/4 motor power, ie 16Kw and 1kw = 17kw of impact energy.

ignoring the motor gives 16kw, thats 7% from the motor.

look at Eds.
3Kj in the disc, 4500watts off the motor.

1/4 second impact, and.....
12kw kinetic and 1100watts motor.
thats 10% of the total energy.

see, its not about the weight or MOI of the internals of the motor.
Hope this clarifies things.