I always use Adobe Encore for DVD authoring and encoding, and that has a pretty damn good AC3 encoder as well.
And I have Windows in the Boot Camp environment, it's just a pain in the ass to restart all the time. Plus, despite the fact that I'm in film school for editing and should probably know how to use AviSynth ... I have not a single clue. I'm not good at all with command-based programs, I need a GUI of some sort that I can figure out.
I can do color correction myself in Avid, so the only other thing I'd really want to do to the GOUT are make it anamorphic (which I have done), and stabilize the telecine judder, which I don't know how to do. I'd rather not go into noise reduction, because despite the insane amount of noise and grain on the GOUT, there's already far too much detail lost due to DVNR, and I'd rather not add to that.