Final report: the general advice was right on. I adjusted my riding style today and it made up all the time I'd been losing. For example, a faster cadence actually felt easier to keep, so I was riding an easier gear at 100rpm rather than one tougher at 90. On the flats, that seemed to give me about the same average speed as before, maybe even a little faster. Downhill I could still use an extra gear (I have a mtn bike triple up front for pulling two kids in the bike trailer), but I found that my tucked-in coasting gets me going a good 2mph faster than with the knobblies. Still working on the right gearing/cadence for the climbs, but again, 100rpm is feeling comfortable and fast.
Now I'm gonna drop the thorn-proof tubes and really fly!