interesting graph. I saw one from a different motor from an EV conversion kit. The best efficiency was at the top end of the torque, not the lower end like this one. I believe that Tesla's motors are similar that way.
Either you are reading that graph wrong or just describing it differently than I would.
The Prius motor efficiency is the highest per that graph around 2200 RPM, which I would describe as nearer the middle than the lower or top end.
It is also more effecient on the top side of the torque area as the RPM rises and has a large blob in the high torque low rpm area.
Tesla's motors are larger all the way around. Size, torque, HP, RPM, everything is larger on the Model S.
Maybe you could show us the graph you saw for the other engine so we could see how it varies from this one?