OK so I got a replacement and had a chance to do a quick run of shots today: I wanted to ensure that it was at least satisfactorily sharp from edge to edge at f5.6 before I got into seeing how it is when shifted. Unlike my first copy, it is indeed acceptably sharp all over. In fact I think pretty darned good. You have to focus it VERY carefully: I think there's some field curvature so you have to make sure that the centre and edge are both in focus in LV within the available DOF (or you have to get lucky!) but get all that right and the signs are pretty promising. Interestingly, though there are mild signs of diffraction when stopping down to F8 or even F11, they are pretty insignificant and you do get a lot more DOF to play with and a touch more corner sharpness at F8.
Many people, including me, have reported problems with this lens on the D800/E but I had heard a couple of people saying it was great. So far, shift tests pending, I am really pleased that I tried another one.
Full sized 91% quality JPEG here: