A focusing screen is available for most enthusiast and pro cameras but not sure if the most popular company "Katzeye" ( http://www.katzeyeoptics.com/ ) has one coming out soon for the D800. They do not have one now.
The AF on the D800 is excellent, best I've used but no AF system perfect in guessing what the user intends. Photographers who think of how the AF system works and what it can and cannot sense, seem to be very happy with the results. There are a lot of people who assume the task of guessing what the user wants should be simple and exact, are seldom happy with any AF system. Time after time when people post missed AF shots, it becomes apparent that the AF system was given ambiguous targets to work with. Yes, MF is the most accurate and is always the final option in any focus challenging situation.
MF was great with film primarily because it was not so common to use extreme magnification as pixel peepers regularly do with DSLR. If ever slide and negative was inspected by a jeweler's loupe, the confidence in one's MF skill would take a hit.