Today I had a go with just the D800E with Sigma 35mm F1.4 ART lens and the A7R with Leica M 35 Lux FLE. Both tripod mounted with delay, and I used a manual exposure that was pretty far to the right but just avoided blowing the RAWS - same nominal exposure for both but of course the actual luminance of the files is slightly different between the two cameras due to the nominal/actual disparities in F stop, shutter speed and ISO.
Nice flat and very constant light.
I shot both cameras at F5.6 and F8 but the Sony I only focussed once, since it has such amazingly accurate magnified live view, a total joy to use compared to the Nikon's dodgy attempt. So I focussed the Nikon frames several times in a focus bracket series, some with LV and some with AF, and selected the best.
I then set the WB to Daylight just to even the playing field somewhat: this was not intended to be about colour rendition, just about resolution.
The Leica lens has a narrower FOV than the Sigma, again a nominal versus actual difference. No lens corrections were applied.
Here's the scene:
Here are the files and they can all be downloaded at full size (converted to JPEG from RAW)
My take: colour shading and FOV considerations aside, I would much rather use the A7R combo: both the lenses have odd field effects but at this distance and these apertures they M lens has them better tamed and is much, much easier to focus accurately and reliably than is the Sigma on the Nikon - and the D800 setup just looks and feels huge in comparison… and the edges of the Sony/Leica frame are in a different league of sharpness...