Indeed if you want more cornersharpness you'll have to stop down to 5.6 or beyond. But the summicron V1 wasn't any better btw.
btw I think your picture is tack sharp Erik, guess it was taken at higher f-stop considering the 400asa?
btw I have edited my first post, since the second shot was taken fully open @f2.8 (I keep records of my shots and just had them checked)
btw on the same roll I shot pictures with a Jupiter 8 which - I just checked them - gives the same soft cornersharpness albeit @ higher f-stops: there might be a little problem with the pressure plate from the Zorki.