I just did an exercise comparing the DXOMark D800 results with a few MFDBs of similar pixel count. I think the closest in pixel count would be MFDBs such as the P645D, P40+ and P45+.
Those are all around 40mp, but if cropped to the 3:2 aspect ratio of the D800, the pixel count should be almost identical and therefore the resolution from all sensors compared should be the same, excluding any significant differences in lens quality, especially if we use the D800E.
I have to admit I was a bit surprised. I expected at least one or more of these DBs to have some qualitative advantage over the D800 in at least one or more parmeter, such as SNR at 18% for example. In fact, on all the graphs the D800 was either the equal, marginally better, or in the case of DR, significantly better.
There was just one instance of the P40+ having an insignificant 1dB better SNR at its less sensitive base ISO of 46. Needles to say, the D800 DR is way better, ranging between 1.3 and 2.5 stops better than all 3 DBs, depending on ISO.
The P40+ is a 2009 camera and the 645D, 2010. That we can get this level of performance from a $3,000 DSLR just 2 or 3 years after these very expensive MFDBs were announced, is very remarkable.