I had a P30+. I also had every full frame Canon aside from the 6D. I now have a D800. I think the main consideration, as compared to a D800e, is how you work, format (4:3), camera, viewfinder, etc.
In terms of IQ there isn't a huge difference between a P30+ and D800e. People will quibble but lenses make more of a difference than the sensor in most cases. The P30+ does beat out the Canons, if you look closely.
I really think we are at a point that the most important consideration is the camera system and not the IQ.
Image quality is so user dependent. If I shot cars, product, non moving subjects, I'd probably be more concerned with detail, but shooting people my first concern is the "look" of the file when first placed in a software suite, then what I can do with it later in post.
I find the dslrs (I have only briefly used a d800 so no real experience there), are more global in color, with hard light can be pleasing, with softer light can be problematic, but it's all subject dependent.
i have a 1dx, bought it because over 18 mp doesn't really concern me and it shoots fast so we can use it for a cut frame look in video. Anyway, like the camera, don't like the file as it's out of the can very warm (especially in lightroom, less so in c-1) and just doesn't look special to me.
I know that someone can show something to refute this and for them, they're probably right, but as I've always said, digital is very user, subject dependent.
The upside of an older p30+ (which I have) is it's paid for and it makes a tremendous file. Really I'd be surprised if I ever needed something more robust in file quality. Since I have my phase backs on older contax, I would like a more modern platform. Looked at and came close to buying an H5d, or a Pentax 645 (still like that camera) even a Leica S, but passed as I wanted to wait and see what/when phase came out with a new camera.
Actually that's not 100% true. I passed because the Leica doesn't have a robust tethering suite, the Pentax shoots and buffers slow and doesn't tether easy, the Hasselblad reviews on tethering were mixed from ok to worse. That left Leaf and Phase. I think the new Phase One's are very high in price, the Leaf seemed better in price, but didn't offer pixel binning and most packages were based on the Mamiya DF which isn't my favorite camera.
But if my studios only produced still projects instead of motion, motion/stills, I'd probably be all over a new back, just because they have come up a few notches in usability.
For us, in the end, it's all about what you shoot and how you want to shoot it. For people that stare at pixels at 100% quality and file size go hand in hand, but for people that shoot, process, retouch and deliver for almost any use, how good a file looks at the starting point and how well it works in post processing probably is more important than actual pixel detail, though the d800 proves that high pixel count sells.
Actually, the acceptance of a digital file has changed so much since I made the transition of film to digital. At first client's always asked for ______ppi, or as they said dpi, non interpolated. Now they just accept a file, without softproofing, usually from our ftp servers and never ask or question original file size.
What clients do notice is the "look", which is one of the reasons so many photographers have gone back and forth from digital to film, to give a different look. Today film is kind of a magic word.
One thing that happened to us twice this year was two separate AD's from two separate projects, looked through our website and selected imagery they both thought were film.
Each image (about a dozen from each AD) we shot with a ccd based camera, either our Phase, prior Leaf or Leica cameras and backs.
Now this is a very small straw poll, but they really believed they were looking at film.
These two images are always selected as a "film" images, even for clients that are not looking in these genre.
Left a p30+, right Leaf Aptus 22
I just think what they saw looked different and different works today.