Don't have any personal experience with a servo motor in a lathe, but I just put a servo motor in my Rikon bandsaw and I'm really happy with it. The motor and controller I have were made as a replacement for the clutch motors used in industrial sewing machines. The 2hp servo motor is about half the size and weight of the 1.5hp AC motor, but I've yet to bog it down even on heavy cuts at slow speeds, while the AC motor bogged down pretty easily. Very slow speeds do seem to be a problem for servo motors, but it could be a limitation of the controller for all I know. FWIW, the torque rating of the servo motor is 16 nm. Don't know the torque rating of the AC motor.