I frequently go out shooting with a friend who owns a Phase IQ180 (80mp).I own a D800 and a D800e. We have compared images on screen and in prints. There are many images where we simply can't tell the difference (except for the aspect ratio). And then there are images where the Phase has slightly better tonality and more realistic color rendition in certain colors. I'm talking VERY SLIGHT. Of course with similar glass and gigantic prints, you will see more detail with 80mx. But it's less of a difference than you would think. Then, there are images where the Nikon looks better. Who knows why? My personal opinion is that for someone who hasn't already spent huge bucks on MF, forget it--the D800 is that good.
BTW, I use the D800 and D800e interchangeably. I defy anyone to pick out the difference between images (same lens/tripod etc) shot with each camera looking at 2 24x36 prints. I've done this test, and nobody I know could see the difference. Sure if you stick the tip of your nose on the paper, you will see a slight difference..but you might not like it better.