From my experience tracking at or below 1 arc secs rms is fairly achievable even in poor seeing for an eq6.
This is a good article for the effects of seeing on tracking Its what convinced me there was some improvment to be had. I have an NEQ6 of a similar vintage and decided to tear it down and do a tune up and belt mod it. I found the biggest impact was actually just replacing the worm bearings. I must have got a bad batch of skf bearings because it wasn't until I switched them out with a couple of hybrid ceramic 608 bearings from ebay I that I started getting sub arcseccond tracking.
So If you were woried about throwing good money after bad that might be a good option to try first. Of course if it doesn't work you could always strip it down again
