I can confirm that you can run the 105 di2 RD and a 11-30 ultegra cassette. I am doing it on my bike I just built up.
You will need to use the ultegra spacing guide and not the 105 one, or just eyeball it to set the b-screw amount to account for the smaller cassette.
Otherwise works perfectly.
You can also run a ultegra RD off of the 105 shifters but its not really necessary unless you want the marginal weight savings. THe shift speed is almost identical on the rear mech, the front is quicker on ultegra, but rear is near identical.