Jddcircuit , cooling seams to be it , I saw that a motor cycle builder is using the Reme motor( very similar to Prius with up graded square wire and probably more magnets ) was getting 200 hp with oil cooling enhancements .
If you look at helicopter gearboxes , they route small tubes to each bearing , gear meshing areas etc and spray oil to lube and cool .
The big commercial utility generators run 5-10 psi Hydrogen
as a cooling gas with a water heat exchanger in the generator box for pulling the heat out .
Helium would be safer at the expense of 1/2 the cooling power of H2 . I would stick with what the utilities have been using for over 40 years .
I like oil and H2 booth . pressure oil for the bearing and H2 to get into the small spaces between the wires . Need to find a way to get the gas into all areas of the winding . I would think using less wire to get cooling channels would end up getting more output .
I wonder if the Prius 2 armature could be raped with carbon fiber for higher rpms .