Small speed variations are easily audible with long sustaining notes, like soprano et al, in some turntables, where the fluctuating note is now getting annoying. If the speed variation is egregious, the overall sound just isn't right. Considering that the mains frequency is never flat, speed variations w/o a controller (like the SDS - and I don't know how good this controller is) are almost guaranteed, but also largely damped by the mass of the platter and the motor itself. A good 'table will exhibit very small speed variations.
An easy way to prove speed variations in any 'table is by actually timing a long enough song (or even better, an entire side of the record) with a clear-cut end (does not fade) multiple times, and you shouldn't be surprised if your measurements are several seconds apart. That, of course, would indicate a bad table. The better ones will exhibit smaller fluctuations.
Whether the SDS does the job or not, I don't know, and if it were me, I would seek to have it plugged into an oscilloscope. But I assume it's a power regenerator which simply amplifies a certain frequency (typically 50 to 60Hz) at 120V nominal output, and then the more interesting question is whether the SDS is better than, say, a PS Audio regenerator (which could also, potentially, improve other components)???
I have not owned a VPI, although I will be buying a Scoutmaster as well as it appears, and from what I have read, the rave about the Scout is more about how silent it is rather than how stable its speed is, and I am sure if the latter were egregious someone would have complained about it. Which probably says, don't be surprised if the improvement with the SDS plugged in isn't as large as you'd have hoped for; this, in turn, implies that the Scout or Scoutmaster are stable 'tables. Then again, let's see what you find out...
On a related note, when I touch the Scout's motor I feel the vibrations, and this not a good thing, as they are transmitted to the platter. I am hoping the Scoutmaster's Black Knight motor will be more silent, and am eagerly awaiting your feedback.