Yeah, it has to be said that resolution falls off as you exceed the coverage these lenses are designed for, even if light doesn't.
It works well for portraits, or anything where it's a mostly central subject, and the periphery is unimportant, or even better for being blurred/vignetted. If you need sharpness to the edge, you kind of have to go with lenses designed for MF: Phase-Mamiya or Rodenstock/ Schneider or Fuji itself, of course. For 3rd party FF lense, Fuji with its 44x33 crop MF makes less of a demand, obviously.
I'll take a look at the 85/f1.4 Sigma, that is impressive coverage. The other thing to check for is Chromatic Abberation. The faster a lens gets the more of that red-green fringing you find when at or near wide open (Longitudinal CA). To my surprise that Mitakon 135 f2 was very good, and the equal of the Zeiss 'Apo Sonnar' which many people think is an Otus in all but name.
(I tried the Leica Noctilux 75mm Leica Noctilux 75mm f1.2 Asph on my Phase the other week, and it has its special qualities, but had bad CA wide open... and the point of this lens is to be wide open! That was a $13k lens!)