Efficiency based on rated amps of belt drive motors does not apply to the 201K2, being a gear drive. The gear drive is so much more efficient since it does not suffer from the drawbacks of belt drives. If you can do it, measure the output at the main drive shaft, not the motor shaft. Measuring the motor output does not take into account the belt tension,and power losses result. At the needle my 15-91 has far more "power" than the belted model.
Leon- I don't have a 201 for a direct comparison, but it appears that the gear case is interchangeable between the 15-91 and 201k2..
Gymnast- You're probably right on the gear count of 10-48. I had no quick way to attach a degree wheel to the motor output, but spinning the handwheel 1 revolution produced between 4.5 to 5 turns at the motor. Hard to visualize the exact locations on the end of motor shaft, but close enough.