I do also own for some reason a LR 5.7 license, but I really can't find anything besides DAM that it does better than C1 Pro 8.
I get more pleasing colors, a more "natural"/more detailed demoisaicing (especially on the files of AA filter less bodies such as the D810), more natural/pin pointed highlight recovery/shadow brightening,... You don't get any of the fake/cheap HDR look often seen in landscape shots.
GPU acceleration for image output is still buggy as of C1 Pro 8.1, but the level of performance with a Mac Pro 2013 with D700 GPU is also impressive.
It really feels like the Phase One team did pay special attentiom to the D810.
Cheers,
Bernard