Originally Posted by
twocicle
My understanding is that the current XTR Di2 Display/junction box + XTR derailleurs + Road shifters provides the synchro ability. I have not read anywhere of issues with the current internal e-tube battery, even with the mtn synchro mode enabled. So if only wanting to upgrade to the new DA shifters, I can't see needing to get the new DA battery as well.
The battery is only needed if using the DA derailleurs (2x11) with synchro mode because those are slimmed down and don't have the logic chip in them which is being added to the internal battery instead.
FWIW, I'll go on record right now to say that I think synchro for road is a total bag of stinking hype. For sure it can be of value trail riding, but for road riding... no thanks.
Ultegra Di2 shifters may work with XTR Di2 derailleur, but they can't provide synchronized shifting. There's a thread at Fairwheel Bikes that explains the various reasons why it's not feasible (bigger gear jump, double-click function, etc):
Shimano Di2 Synchro Shift for road bikes? - Fairwheel Bikes Blog
For tandem teams who are happy with mechanical shifting, synchronized shifting is not a compelling reason to upgrade to Di2. However, it's a reasonably affordable upgrade ($200-300 for battery and junction) for those who already own 11sp Di2 or plan to switch in the near future. We'll have plenty of time to weigh the options while waiting for the spring 2017 launch date. I don't see any downside for Di2 to coordinate the front and rear derailleurs for fast & smooth gear changes.