I can't believe I haven't done it yet, but I want and need to do some tests to compare image quality from 5D III vs a7R. Per my comments above, I typically use my TS-E 24 II on my a7R for single hand-held exposures. But, if it's on a tripod and AEB brackets I like using my 5D III.
You're right on Charlie - I've noticed that too. While I love the a7 level, I too have noticed it's less precise than I want. It annoys me to get the camera "level" and be able to rock the camera around and it's still "level".
I had done a quick (and by far only) side-by-side comparison of 5DIII vs A7R late summer, but that was conducted within a week after acquiring the latter and I was still learning to operate it. Hence, I'd say it was not a valid testing. I set up 5DIII with TS-E24 on a tripod, then swapped with A7R and remounted TS-E24. Somewhere between 2-3 minutes have elapsed between the two shots. At the time I relied on focus peaking but since then I have used zoom focus for a more accurate focusing. As a result, A7R shot is slightly softer than the one taken with 5DIII. On the other hand, the A7R shot did capture more details and when manipulating exposure during post processing, it brought out more detail from both overexposed and underexposed areas. White balance is another factor I should consider; I shot both cameras with AWB setting, but I should actually set it to a specific Kelvin to level the field. By the way, Canon's AWB setting was closer to actual condition at the time of the shot. It definitely warrants another round of side-by-side comparison.
Re: in-camera level: 5DIII is much more precise. In some of early A7R arch/interior shots, I had to correct foreshortening in the post because I believed its in-camera level. I have since relied on my eye and grid display on the LCD to make sure the camera is as level as possible.
Both examples shot at 1/160sec f/8 but A7R shot was somehow set at ISO 64 - it's the pesky directional controller that is pressure sensitive as opposed to dial like that on 5DIII - so I had to +1/3 exposure to bring it as close as possible to the 5DIII shot, set at ISO100.
5DIII + TS-E24
A7R + TS-E24
