I've just setup the new BT-DN110 battery on my Trek Domane Di2 and checked out the Synchro Shift.
I have Dura Ace FD-9070 front Derailleur (and Dura Ace 50/34 Crank) but Ultegra 6870-GS rear derailleur and shifters, with an 11-32 cassette. I already had the EWW01 Wireless unit and didn't need to upgrade that, just the battery.
Really easy to upgrade and everything seems to work OK, just two little glitches so far, both of which I see others reporting here.
Tapping on my Garmin 1000 to change the shift mode shows updating, then leaves the mode unchanged, as others have reported, so I guess this is a problem with the firmware (I have the latest at both ends), either Garmin or Shimano, as the communication is breaking down between the two. Double clicking the button on the EW90-A junction box works fine, and the Garmin beeps and shows the new shift mode.
Finally, I have tried to customize the shift pattern for Full Synchro Shift, and it is all possible apart from I can't move the auto shift from the big ring to the little ring from the 28T cog to anything else apart from the 32T, I wanted to move it to the 25T, but for some reason all the other cogs are greyed out! I have tried moving the target cog up and down, but it makes no difference at all, only 28T or 32T can be selected. I see others report much the same thing, so either Shimano think there is a reason to stay in the big ring until 28T or they have a bug.
I've moved the switch from the little ring to the big ring down one cog to 14T, switching to 18T, as this is actually a smaller gap and means the change in front ring is delayed and possibly avoided. Reducing the number of front changes seems to make sense, as does making the ratio's slightly closer.
I wanted to change the shift to the little ring for the same reason, and to avoid having the chain quite as crossed (50T/28T) so hopefully Shimano will fix whatever is making this not possible.
I have left the button switches mirroring a mechanical setup, so I don't have to learn a new habit and can switch between bikes, but will only have to change the right hand up and down in full synchro shift. I still have the option of a manual change of the front ring, with the left hand. If I find I'm doing this a lot, I will give Semi Synchro a try, as shifting the rear 2 to match is something I currently do manually.
Well done Shimano for retro-fitting this to the older kit. They did get a new battery sale out of me, but I guess my battery would have needed replacing eventually!