I have a simple entry-level dropweight stringer as well. I've been considering upgrading too, and right now I'm eyeballing a Gamma Progression II 602 FC. I don't mind the dropweight. It's a constant pull technology that doesn't need to be calibrated.
From what I've read, the single biggest things you should look for when upgrading your machine is going from flying clamps to fixed clamps, as well as a more stable and rigid base to mount your racquet (to reduce deformation).
Everything else that comes with high-end machines are primarily for those who string 10+ racquets a day: time is money, so speed is of the essence.
But to string a racquet as accurately as possible, then you can't really get any better than a constant-pull machine with a rigid base and fixed clamps.