Somewhat OT: One thing I've noticed via using cameras, like Sony's A7r, that accomodate precise focusing in the extreme corners is that corner "softness" is often a matter of field curvature rather than resolving ability. Such lenses, which may require stopping down with manual focus (central split-image or microprism) or D-SLR (no corner focusing points) cameras when the subject is well off-center, usually turn out to perform fine at wider apertures when the focusing point is placed precisely in the frame over the subject.
-Dave-