Crane550 made a 2 or 3 to one chain drive speed increaser and used some relatively massive ceramic magnets.
In general, neglecting the efficiency and power density improvements that come with larger machines, the "performance" of a motor generator is approximately proportional to the energy density in the magnet. 10x ratio between ferrite and ceramic.
I just measured approximately 216 watts out of a ferrite magnet hvac fan motor at around 700 rpm, 50 volts at 2.5 amps 3 phase, about 3.5nm torque going in. 12 poles, 18 coils, frequency was 78hz out
The magnets are about 2.6 inches long, 3 inch outside diameter, .3 inches thick. That's like 6 cubic inches of ferrite, for 250 watts at 700rpm at 86% efficiency.
Cogging torque is .25NM
If my rpm was actually 780 then its significantly less efficient than i thought.