This isn't exactly what you are asking for, but I did shoot the same image (outdoor architecture, all artificial lighting, night shot) with a Canon 5DII and an Aptus 7 (33 MP) on a tech cam with a 35 mm lens. The Aptus image was clearly superior, especially in highlight detail. Where the Canon had clipped channels and made the wood go yellow, the Aptus kept both accurate color and impressive detail. It was good enough that if you needed a bigger image, you could just uprez it in photoshop as all the useable detail in the scene was captured. I was impressed.
The client didn't see any appreciable difference and went with the Canon shot because it was a little wider and they liked the 2x3 perspective. I vastly preferred shooting with the Canon because the camera was mounted on a very tall tripod and where the Canon could be completely controlled with Capture One, the tech cam required making adjustments to the lens/shutter blind by reaching high overhead. The tripod was on a hill in a snow bank.
Now, I have not done a similar shoot with a D800 or A7/r, and that could be interesting. Especially an A7/r with a Canon T/S lens.
I got to play in a studio with a model and a D800 and IQ180. All the picture from the D800 were technically good, but I had a hard time getting a great picture from it. It just seemed a bit flat. It took all my C1 mojo to make them look good. The IQ180/DF (might have been DF+) was a different story. Getting an acceptably focused image was very difficult because of the DF and full size chip, but the ones that were in focus were spectacular and presented a very different challenge in C1 -- the images could be made to look any way that you wanted. While the DSLR had IMO one correct way to process them, the IQ180 files were so flexible that your own creativity was the limiting factor.
What's the point of all this? Well, if you are looking at legacy backs, I think a bigger sensor is a better sensor as long as you don't need the advantages of a smaller sensor (higher ISO, capture speed). The more recent digital backs (P30+, P40+, Aptus 7, Aptus
are more color accurate than any DSLR. There are limitations of the DF body, especially with focusing. This is more of a problem with a large chip because of DOF. The digital backs (any of them) tether better and more reliably than a DSLR.
So, from an overall technical view, I would say there are advantages, but at a cost. To me, the real reason to invest in a 22 mp back is because you want to upgrade to a newer technology back at some point, and want to start building a camera system. I would be surprised if there is any advantage to a 22mp back over a current DSLR. When you move up to a 33 - 39 mp back, I think that changes.
