Well I'll mention some other ideas, FWIW.
Atmospheric seeing conditions are around @2~3 arc sec's at best (guesstimate avg)...so you cannot RA track/guide better than this. By track...lest say if you are RA tracking at perfect sidereal rate...the stars will still oscillate around regardless.
TDMs are expensive...auto-guiding is cheapish and can attain good results with guiding. I'm not sure what CGEM users get but lest say around +/- 2 arc seconds with PHD.
Seeing will be on top of this as it would with a TDM.
TDM works in RA only...what if the polar alignment is "off"?
IMHO...the best and far cheapest option is off axis auto-guiding.