freebeard's right about the voltage.
I started this thread to brainstorm ways to get a bit more power out of it.
Generally, the RPM is limited by the rotor's strength and the BEMF generated at the high speeds. ORNL has tested a similar Prius motor to about 13000 rpm. Problem is boosting the voltage over the 650V, may just allow a higher rpm or top speed, but I have no idea what the rotor can handle. However, this won't boost the output power.
I'm very curious how much a lower battery voltage will limit the motor speed. I'm going to start with my Leaf battery pack, and hopefully learn something useful from that. ORNL did some tests at 500V and 250V for the Prius, perhaps that might help.
Total output power is directly tied to the current the stator can handle. The current creates heat and how the heat is dealt with becomes extremely important. To me, one of the most limiting features of the MGR is its cooling. Basically, it relies on the air blowing over the case to cool it. There is internal oil flow designed to reduce hot spots in the stator windings, but the heated oil has nowhere else to go.
I've been working on these issues with a Siamese twin motor setup. It's getting close; the weight is near that of two OE units. The new design uses all the 'Yota gears, etc, and will be prototyped on my mill. There is an internal gerotor pump to circulate oil to an external cooler and provide lubrication. The unit is completely symmetrical, which should make mounting easier. Also, the pump can be reversed so that the motor can be mounted in a behind the axle setup.
My evil plan is to use two of these siamese units, totaling 200kw+ in my eclipse.
If the cooling system works properly, I'm sure I can get another 10% or 20% out of it.
Should this work out as I hope, then I'll start work on a Prius based 120kW unit.