I Acceleration on an electric unicycle, how much does the rider have to lean?

[A.T.]

It is the deflection angle from the vertical, of the line from the contact patch to the centre of mass, that decides the acceleration necessary to maintain or reduce that angle of deflection.

And adding weight to the upper body, that is strongly leaned forward (see video), will move the centre of mass forward, increasing that angle of deflection, and thus the necessary acceleration. The only problem is maintaining body tension under additional load, hence the railing for support.

Increasing the mass will increase the power required from the motors to stabilise the situation.

I was replying specifically to the scenario, where there is still more power available:

For the P6, the limit is the amount of torque the rider can exert onto the EUC.