Long story short- I have driven TDI's since... Forever, and rack up LOTS of miles on them.
I purchased 4 2015 TDI's (2 Passats, a Golf Wagon and a Beetle Convertible), after the phase 1 fix, and the repeal of the "Stop Sale". I sold back 2. (2011 and 2013 Jetta Sportwagens), prior to buying the 4. I know how they feel when they drive.
I have LOTS of pre-and post- Phase2 fix miles to give some great data.
I drive Uber with one of my TDI's. 200-400 miles per day 6 days a week. It's been 20k miles since my phase 2 fix, and I've noticed no decrease in MPG's, and a slight change in shifting between 2 and 3rd gear. The engine seems to momentarily lag after the shift, as though it hit an air bubble in the fuel line. (It hasn't).
Phase 2 also reflashed the DSG transmission, so It's worth noting the change.
Also, I've noticed My DEF (Ad Blue) use is marginally higher, but I still have no problem making it to 10k oil change interval with the Ad Blue. (Although now my 1500 mile DEF countdown starts around 9000 miles after topping off the 5 gallon DEF tank, whereas before it didnt come on until I was right at 10k miles since adding it.
Pre-Phase 2 miles- 58,000. 40 mpg combined driving.
Post Phase 2 miles 19,800- 41 mpg combined driving
Best tank- Pre-Phase 2 fix- 810 miles. Alabama to Colorado all- interstate drive.
Best Tank Post- Phase 2 fix- 833 miles Alabama to Colorado- all-interstate drive Given that the engine had more miles on it, I'll pretty much call this a tie, since the looser engines always give better mpgs.
Interesting fact: the new phase 2 seems to like higher cetane as I've found that Bio-blends and "Cheap" diesel (Racetrack Station, Pilot Truck Stop (B-20 blend) netted 38 mpg several times, Whereas Shell Diesel gets over 42 every tank.
Adding 8 ozs of Power Service fuel treatment adds up to 4 mpg as well.
Overall Impressions:
Phase 2 fix engines seem to like higher cetane, and better quality fuel.
They use just a bit more DEF.
There is (maybe) a lag in 2nd to 3rd low-speed shifting, but Sport and Manual mode don't experience this. EDIT: 8/5/18 It seems that it's a very rare occurance, and only under very specific situations. (almost putt-putting around scanning downtown streets for Uber Passengers). I don't think it's a real issue.
Overall-- I'd say statistically, all things considered- NOTHING has changed when comparing pre- and post Phase 2 performance and mpg's.