I continually wrestle with the question: when are you trying to put too much cartridge on your turntable? The old Linn hierarchy (turntable, then arm, then cartridge) is valid IMHO, but it is not an absolute one.
One thing is certain, that speed control counts for a lot, and you will hear it in ways that are far more subtle (and yet equally important) than merely gross characteristics like wow and flutter.
With each improvement we made to our drive system, we heard better top end extension, smoother highs (not a paradox), tighter, and yet more harmonically dense bass. This last attribute may seem a bit odd, but take this in conjunction with the extended highs, and it will make sense to you. Bass harmonics are after all, upper frequencies overlaying the fundamentals.
A while ago, there was a review of the Rockport Sirius turntable on International Audio Review (http://www.iar-80.com/page12.html). If you can wade through Moncrief's wordiness and redundancies, his point will bring this concept home ... that minute speed instabilities will show themselves as FM distortion ... kind of like the analog equivalent of jitter. This is the reason that getting the speed right will smooth out and extend the highs.
Here is a provocative quote from that review:
"Moreover, the distortions due to turntable speed errors could actually be more pernicious than those due to cartridge imperfections. Cartridge distortions tend to be amplitude distortions, thus producing harmonic distortion (some of which is actually psychoacoustically benign) and amplitude modulation distortion (due to the fact that music, being complex, contains many frequencies at once). But turntable speed errors predominantly produce a different kind of modulation distortion, called frequency modulation distortion. Pioneering work by Paul Klipsch 50 years ago already found that this FM distortion is more pernicious than AM distortion, being more audible and more objectionable in smaller amounts."
Know that as I write this, I have stopped selling motor controllers separately ... it takes me too much time. I state this from the perspective of good sound, and not selling you a controller.
The question of course arises as to whether your manufacturer's drive system upgrade is a significant one. This is a question I cannot answer for you.
What I can state unequivocally is that a drive system UPGRADE will yield enormous returns. Coupled with the fact that your current cartridge is to your liking, I'd seriously investigate a drive system upgrade.
Thom @ Galibier