I'm no technician, but I owned that P21+ back, and it's a true joy to work with. I think when people talk about this mystery difference between MF and 35, they're probably seeing differences between CCD and CMOS mostly; maybe there IS something going on there. For sure, the default sharpness. I doubt about bit depth.
All I know is, you throw that P21+ on your camera, and you can shoot very fast, almost the same feeling as having a Canon in your hand. And you have the complete integration with CaptureOne, if you need to tether, (unlike the 1ds3).
If Phase rigged up a great LCD onto that P21+, and hit a good price point with them, they'll sell them like hotcakes. It's the nicest back I ever owned. Trouble-free, and faster than you'd ever need. Everyone laughed at me when I bought one -- "Why don't you just buy a Canon, if it's that small a file?" -- but if you want use of some of those nice MF lenses, it's a much better choice.