I checked to see if the soft corners wide open on the RX-1 were because of lack of field flatness. When I focused on a corner, the corners were very marginally sharper than when I focused on the center. However, the center was greatly softened when I focused on a corner when compared to when I focused on the center. I conclude that there is some unflatness to the field of the RX-1’s lens, but that the wide open corner softness cannot be blamed on that.
By the way, the focusing ring action is every bit as fiddly and unsatisfying on the RX-1 as it is on the NEX-7. It’s possible to get things sharp with manual focus, but it’s not much fun, and it’s not nearly as fast as focusing a lens in which the focusing ring operates a helicoid.