Where in the frame does the A7R image not impress you? The pixel pitch of the 7D is smaller than the A7R, so the center of the image on the A7R should look as sharp as the 7D image (assuming it's in focus). You think the 6D images look better than the A7R; is this only when pixel peeping at 100% resolution, or when viewing a full image at the same size?
Depending on what you're seeing I see two possible explanations:
1. The lens is sharp in the center and slightly soft at the edges. When using the 7D you're looking at a crop of the center, so everything looks good. The 6D is lower resolution, so the edge softness gets hidden to some degree and you don't notice it. The A7R is high enough resolution that when you pixel-peep you see the edge softness. If you view the 6D and A7R full images at the same size they should look similar.
2. Issue specific to the A7R - either the A7R images are not completely in focus, or the adapter/lens mount has a little bit of tilt (this wouldn't surprise me with how heavy that lens is). The A7R doesn't have the best autofocus, especially with an adapter; it's possible the images just are not in the best focus.
If your problem is #1, then the 5DIV or especially 5DSR will show similar issues to what you're seeing on the A7R. If you problem is #2 then the 5D images should look a lot better.