A7/A7r shots with Pentax lenses will be the closest thing that there is.
The only potential diff is the sensor stack thickness that is used.
The one used by A7 series is rather thick and causes edge issues with some lenses (esp RF ones with recessed rear elements).
IMO, 24mp FF, still ok for a lot of legacy lenses.
36mp it gets even more critical and even Nikon had to release a list of recommended lenses that would work better on the D800.
Problem is the off center, edges and corners.
The old lenses either strike the sensor at an acute angle or have a different focus point to the center.
Old UWA are usually not as good as new UWA.
28mm and above, generally no issues. (plenty of good examples of all sorts of legacy lenses on A7r)
FA Limiteds are fine.
So are most FA*
In fact most 28mm and up are fine.
Some did surprise me though like the FA*24 which I find is optimized as a larger aperture close-people distance type lens rather than a lens catered for infinity landscape work.
Wider stuff, field curvature is common (not uncommon for old wides of any brand and even on some modern ones).
All said, that does not mean the lenses are that bad (YMMV)
Firstly, the composition is usually not on the corners.
The human attention is also usually not on the corners or edges (so long as they are not so bad)
Neither are most compositions placed on the corners (though off center in the thirds area, I feel it needs to be good enough)
Very often, we go for other aspect ratios (5x7; 8x10; etc) and those edges/corners are left out as well.
The famous Canon 17-40/4 L is rather well known also for the poor edges and corners, but its been a great workhorse for Canon for so long with so many great photos taken with it too.
So lens review numbers is not always an absolute, in the field.
I have shot quite a few legacy lenses on the A7.
http://www.clubsnap.com/forums/showthread.php?t=1475299
http://www.clubsnap.com/forums/showthread.php?t=1488003
My Flickr album for Pentax Lenses on A7 -
https://www.flickr.com/photos/29329237@N07/sets/72157639381818034/
https://www.flickr.com/photos/29329237@N07/sets/72157639220201684/ -
this one I did initially, then gave up as I realized that a lot of the PF ppl in the FF section were only interested in the 'big theories' and 'conspiracies' talking about a FF and not on the facts. Anyway, I found shooting the stuff in sequential f2-f8 format rather boring
This one has full sized jpgs.