Our setup - we're newbies to Di2:
Ultregra Di2 with Dura Ace caliper rim brakes
Gearing: 52t/34t front; 11t-36t rear. The front it beyond Shimano's spec of maximum 16t difference between chainrings. It shifts reasonably well but the jump from 52t to 34t is huge. We're pretty reluctant to go to a narrower range (like 50t/34t). We did a ride on Sunday where I wished we had a higher top end.
K-Edge arm on the rear derailleur.
Did not have display when we bought it but have since added it. I think this is very helpful.
We have not done any rides that would tax the brakes. Our old tandem had rim brakes & they were adequate for most every ride (including Mt. Evans).
I was skeptical of Di2 at first because it dictates only two chainrings and I found myself mis-shifting all the time compared to mechanical shifters. However, the more I ride it, the more I like it. I'm still not in love with it because of the compromises it enforces (note that I haven't said anything about the cost).
Maybe when they introduce synchroshifting, this will reduce my complaints about the big gear jumps.