I just lined up the K-1, D810, and Canon 6D for some pixel peeping.
Some interesting things, IR rates the K-1 a little lower for resolution on the chart, but lining up the high ISO test The K-1 just kicks the poop out of the competition, appearing to be a stop better than the D810 and of course, I threw in the 6D because of it's low light reputation, and it also get's it's but kicked.
Pentax K-1 Review: Now Shooting! - Exposure Nikon D810 Review - Exposure Canon 6D Review - Exposure
The sony A7s maintains better colour at high ISO but the K-1 maintains far more detail. I'm guessing the K-1 can be brightened up in post.
Sony A7S II Review - Exposure
The D810 is measured at a bit higher lw/ph I'm guessing because of the availability of 32 and 64 ISO, although I really can't see much difference between the D810 and K-1. One brief check of the K-1 and A7rII at 3200 ISO has me thinking the K-1 is probably better than the A7rII for low light noise.
Altogether, impressive results.
I went to compare the D810 with the K-1 both at base ISO's and unbelievably, the cards at the bottom right clearly shows that they missed focus in the K-1 image.Apart for that corner, the K-1 image makes it pretty clear, A7II or D810 doesn't really matter. You can buy a K-1 knowing you're getting the best, both in terms of the clarity of the image and the high ISO performance. Though from 800-6400 ISO it appeared to my eye the K-1 had about a 1 stop advantage over the D810 in the still life image, and maybe a half stop on the Sony A7rII.