Tough stuff.
That was sarcasm. Going through something note-by-note is the best way to be sure that the pronunciation's at it's best, or near it's best. Utilities are best used for repetitive, factory-assembly line stuff, which editing a .UST should not be. Along the way you'll have the opportunity to catch errors like bad envelopes, a missing phenome, etc. that will be harder to catch if you use a machine to convert it.
When I know that I'm going to be shoving a .ust through the resampler a few times for a while I'll usually put music on at a low volume to keep myself from dying of boredom.
If doing the whole thing in a sitting is grating on you, then chip away at it slowly - don't burn yourself out, don't let yourself get sick of it. The more time you spend on it, the better it will sound. And days in-between help - I find that when I'm working on one thing for a while over a day I tend to obsess over the wrong things while overlooking other, more noticeable, issues. If you spend a whole day obsessing over something then I don't think it's too much of a stretch to guess that you're the same way. It ain't a good way to be.
Good luck with the thing you're working on >__>