Well I managed to have a short play with an A7R this evening with my Zeiss 21 attached via a metabones adaptor. Probably didn't have enough time to play with it to be honest. First impressions are good although the menus are a bit complex compared to the D800E. IQ wise very impressed, certainly on par with my Nikon from the 10 odd shots I took. Another thing I noticed is that the corner colour shading I get on my Nikon with the Zeiss is completely eliminated which is a bonus. Still not sure how the focus peaking works so will have to read up a bit more - seemed to have quite a bit of tolerance - even at f2.8 close focus, some of the far background was shaded in red. Its a good camera but so different design wise it almost feels like a toy.
My biggest concern is that it feels like I'm downgrading........this is a problem for me. Need to have a bit of a think to be sure.
The menus are a nightmare I agree. The good thing is the amount of buttons that you can customise to nearly anything you want, unlike Canon who say this button can be this feature or this pretty much.
The corner shading is interesting, I was going to say is the auto lens correction on in camera but I don't think Metabones communicates for that information. Maybe it is a benefit of mirrorless that I am yet to discover due to the flange distance or something else?
Personally with regard to focus peaking I am not convinced it is for us landscapers. I have assigned it to a custom button, set it to 'mid' and 'yellow' then also set another custom button to change the picture profile to B & W to make it more visible then tried using it as an aid to establish what aperture to use for required depth of field. Then turned it off, and changed back to colour and magnified near and far points of focus to check critical focus. Which for me so far the focus peaking has not achieved...(Also as you stop down diffraction possibly kicks in and reduces the amount of high contrast edges visible and the focus peaking seems to drop off in certain scenes?)
Do you use a depth of field preview button on your Nikon when focussing Neil, or do you focus wide open where you think it should be then stop down after? Bear in mind the Sony will be stopped down to whatever aperture you are at anyway. One cool feature with with the Sony is (assuming you are not shooting shutter priority) if you want to zoom in to a dark bit of the scene to check focus but the monitor brightness is set a bit low (to save the rubbish battery!) you can just up the exposure compensation to brighten what you are seeing and make it easier to check. Then hit the 'disp' button to scroll onto the live histogram and set it correct. Coming from Canon this is an advantage because if you have exposure simulation previewed when you focus with the dof preview button depressed it all goes dark. If you don't have exposure simulation enabled the screen nicely brightens automatically as you move around zoomed in but you don't get live histogram!
I actually think the camera for you may be the D810 if it has better live view implementation? I don't know much about Nikons but it may be similar to the 5d2/3 where the IQ is very similar but the 5d3 gets a better screen making it more usable in some ways.
The only reason to go Sony is to save weight. Which, let's face it the D750 may achieve (if it has good enough live view compared to your D800) as without the extra batteries and lens adapters will be the same weight anyway.
I'm pleased with my Sony, but then I have not had this sort of dynamic range before, the files are superb. If you are after an IQ upgrade it seems you will have to save up for the A7r2 to really enjoy that benefit perhaps?