My personal experience with an outboard speed controller is limited to my purchase of the Basis unit for a Basis Debut table. This is a quite expensive dual phase unit that required modification of the motor (two separate power feeds to the two-phase motor and removal of the phase splitting capacitor). The benefits of using this outboard supply is, to me, quite subtle. While the sound is better--a sense of a more "solid" sound--the improvement is not nearly as obvious as the much cheaper upgrade of buying the newer and better belt offered by Basis. The Basis table, being a premium unit with incredibly close tolerance machining, probably had reasonably good speed control so that only small improvements could be made anyway.
I would say that while a better power supply for the motor will probably improve sound, I would expect a low bang-for-the-buck return on such investment. The ability to change speeds without moving the belt is a convenience that could factor into the decision.
