So I've now been playing on my newly fixed-up horn for about a week. It feels very different, particularly when I first got it back, but I'm becoming acclimated to the differences. The biggest change is the feel of the keys: everything got moved and adjusted, along with new pads etc. Key throws are shorter. The keys are quieter and tighter. It's taken some time to get used to, but it's absolutely better. In particular, with less distance and slop in the key motion, I'm encouraged to have less distance and slop in my finger motion. This should help my technique.
Keys were not the only changes. New pads everywhere, a bit of fresh oil in the bore, and very careful sealing. The boot seals down to 8 on his magical pressure tester, which is very good. (What the units are, I have no idea. The test is basically how much positive pressure the joint can stand, though.) I was expecting the improved seal to help low register response in particular. And it does, but not as much as I was expecting, demonstrating that I still have a ways to go on my reedwork. What I didn't expect was changes in sound and intonation. There are lots of individual notes which are improved: for example, F#3 and B1 were previously relatively bad notes, much brighter and more nasal than the notes around them. They now fit better into the notes around them. That's probably due to oiling the bore, he said. And I think intonation has improved: things like low D2 and E2 improving makes sense, given the sealing, and adjustments to pad height, but why would notes in the middle register (like B2 and C#3) improve? I don't know, but I'll take it. The whole thing plays better, I think, than when it was new from the factory.
All in all, a great improvement, well worth the trouble. Now, it's all up to the reeds and me.