Finally received and installed the K3 focusing screen (type used for Nikon FM3a) that was customized for my Nikon d750. Installation was a bit nerve racking with reinstalling the focusing screen shims, but finally got done with no scratches, dust or fingerprints on the screen/viewfinder.
The net result is that manual focusing is now much easier and a real pleasure. I carefully tested the focusing accuracy of both AF and manual focus glass using a tripod. I took three shots each of several flat, yet detailed subjects using my 28-70 AFS at f2.8 and manual 50 f1.2 Nikkor (at f1.2) using (1) viewfinder manual focus, (2) Live-view focusing (at maximum magnification) and (3) using the autofocus (with the 28-70) or the focus confirmation dot in the viewfinder (with the 50 1.2). Viewing the photos at maximum magnification, I couldn't see any real differences among the photos using the same lens, with all showing a high level of sharpness and detail. So the focusing accuracy looks pretty much perfect.
All viewfinder displays continue to work perfectly and the metering system is not affected at all, while the DOF effects are more evident. Basically there are no operational disadvantages to this modification. Of course there is the cost $92 from focusingscreen.com and the care and nerves needed to install the thing without damaging anything. Overall, highly recommended.