Originally Posted by wgscott

In the case of Ultegra hydraulics, the calipers are the same as XT. (I have a warranty-replacement of one of my Ultegra calipers and it actually is an XT caliper. The only difference is one has a slot-head bolt to hold the pads in place, and the other has a hex-head bolt.)

I don't think I want to spend the money for Di2, but you can in principle use road controllers and mountain derailleurs (as long as both front and back are the same, like both are XT). Your Wolftooth idea sounds much more cost-effective.