As others have said, the 5d3 is like a 5d2/1ds3, with better high ISO performance. It is, to my mind, a small 1ds3 without the 1ds3 finder (which is really nice). Its a steroidal 5d2, a much better camera mechanically than the 5d2, with slightly better IQ. It is great for shooting lifestyle and portraits, and journalism. I think it would be too far a drop from the P25+. The D800e is a huge improvement over the 5d3 in terms of IQ, at least for landscapes, not so much for people.
The Mamiya 80AF is a very sharp, very contrasty lens. It does well with 33 and 39mpx backs, in my experience. You may want to check out another copy.
I would also suggest that, for less than twohundred USD, you look at the Mamiya 55mm manual focus lens. One of the best lenses I've ever used. The 45mmAF is good too once its stopped down. This lens used is in the fourhundred USD range.
Other than that, look at a D800e. Its sharp and all that.
As to C1, if it bogs its probably due to your video card. Runs like a champ on my 2012 iMac, 2011 MBP, and even my ancient 2006 Mac Pro, although I have a much improved video card.