Synchro Shifting thoughts
Regarding Synchro Shifting from shimano:
I have similar set up and subsequent problems with the timing and compensation shifting. it would catch me out about 50% of the time. Once you are used to the warning, your brain will just ignore it. I found it to be far more irritating than helpful. I can see the benefit in mountain biking or with close ratio chainrings.
I have XTR drivetrain with str785 shifters. I reprogrammed the whole thing like eTap. large right paddle is rear der down cog, large left paddle is rear der up cog and so forth. I run this in manual mode. As long as the chain can go to the 52x42, I can't break it (and the standard long chain from SRAM works out of the box). In addition, the XTR system will not allow the small ring to go into the 11/13/15 cogs, so slack chain problems are avoided.
this setup is more intuitive and any gear changes are my decision, not the machine.
I do find that the proximity of the small and large paddles are too close and (I) will occasionally cause the wrong shift, so I might add "sprint" shifters and reprogram the front derailleur to these only (like campy thumb shifters), leaving the brifter paddles to only shift the rear derailleur.
so eventually I will have a road/mountain Di2_eTap_Ergopower 3x11 monster drivetrain.
MG