Happy with the fix
I'm happy with the fix. The car feels a bit zippier, if anything, and makes a bit more noise. The diesel mileage is about the same, as far as I can tell at this point (~1500 miles since fix).( Update: average consumption for 2017 was 34.8 mpg. Average for the first five refills after the fix is 34.4 mpg.)
The DPF cracked shortly after the fix, so the car now has a new DPF and EGR filter. I drove a 2015 GSW TDI as a loaner, which seemed surprisingly lifeless, until I realized it's geared 15% longer. Somehow I had overlooked that being mentioned before; or maybe it wasn't mentioned? That cured any remaining doubts regarding fix vs buyback + buying a 'new' 2015.
Regenerations have been spaced farther apart than before the fix, but the pattern is still a bit irregular.
Added: the 'S' mode shift pattern is now more useable in my opinion; it no longer stays in a low gear at low load unnecessarily. Before it would rev above 3000 RPM even if your foot was already mostly off the throttle. I think the 'D' mode shift points got a bit sportier in exchange, but I'm not sure. But 'S' mode is definitely more useable for me after the fix.
And a comment on mileage: my lifetime average is 36 mpg, so I never got those 40+ mpg 600 miles tanks like other people. My average has also been dropping slightly over the years, probably due to highway speeds increasing from 70 to 80 mph in the Chicago area ( not that I would ever go that fast).
I'm perfectly happy with the 36 mpg, given the speed and performance I'm getting out of the car.