I would expect it to be in the region of a few hundred but have never tested it.

No, shorting the windings to ground does not create the high voltage spikes as the magnetic field is not collapsed, which is what happens if you open the circuit.

I expect the Series regulators will have some form of TSV protection, either that or it's FET's are rated to survive the spikes, the problem with this is the spikes are still created and the stators' own insulation will not be rated to that voltage level.

Open circuit voltage is a few hundred - emf from breaking current may well be in the order of a few thousand. Transistor assisted contactors don't have to put up with that.

 

Open circuit voltage is a few hundred - emf from breaking current may well be in the order of a few thousand. Transistor assisted contactors don't have to put up with that.

Initially I thought the same but these motorcycle stators don't have that many turns per phase, it's more about current than it is voltage.
According to the manual, open circuit voltage is only 80 VAC (at about 5,000 rpm). Based on that I doubt it would spike above a few hundred volts if opened mid cycle.
However I have never tested it or seen any test data.