400Hz is also used for radar and com. gear. (Aviation and Military) From what I understand, it offers a much smoother wave form when rectified for those uses with a higher available current. (as the alternation spends a lot less time at lower potentials)
FYI 50 and 60 will give you a kick in the pants, 400 will burn you in the proccess.
As for generator shaft speeds, they run roughly about the same general speed as 50/60 gennie sets, they just have more magnetic fields on the stator so the armature crosses more fields per RPM. Or at least the sets I have worked with... I assume a convertor (I haven't used any) would operate just like a rotory phase convertor, instead of just adding a phase, add a bunch more fields to cross.
