Very unscientific testing indicates that going back to stock did exactly what I expected.
First thing I noticed was that it feels smoother and less "punchy" at very low revs. That jives with both softening the throttle response curve, and less aggressive timing advance. Having played with timing advance on older cars where such things can be manually adjusted, and on cars with programmable ECUs, this all makes perfect sense and it's what I expected.
Fuel economy is up. Way up. More than I expected. Made a quick trip out to the mall last night in the rain. Without really trying, I was hitting 42 mpg by the time I got to the mall. Puttering around the mall parking lot looking for a parking space got it down to about 39. Leaving the mall after dinner got it down to 36. And before I was back home it was over 40 again. That's with several stop lights in both directions, very wet roads, and again... I wasn't trying. I'd call that a pretty solid 3-5 mpg improvement in "easily achievable MPG". Not bad at all.
Went out again today to get lunch and round up some car parts. Similar trip. Not quite as wet. Worse traffic, and more waiting at stop lights. Totally not thinking about it, as I had a friend in the car and we were chatting. Very similar results.
Oh, and all of the above was with the AC on.
Next time I drive it, the Volo is going back in. I've already reprogrammed it with economy settings. Should be interesting.