Update: how to focus this lens
This is really complicated. Really.
Talking about shifts, rise in particular and in landscape orientation:
I have shots where the top is in very good focus at 10 rise and F8. And this is on a planar subject (a building in front of me to which I am well aligned). Repeat: there is NO significant edge weakness even at extremes of shift. However, to get those sharp tops you have to focus in the middle of the top third of the frame in LV, and then the bottom third of the frame looks soft. If you focus in the centre of the frame you get an acceptable level of sharpness throughout, judges at 50% on scree, though the top is less sharp than before.
At five rise, focus in the same way, maybe a touch lower, and all is good.
At no rise, focus on the top third line and you're about right.
Now switch to a distant landscape. WTF! Still at F8, focus on the centre of the frame and the edges are a bit fuzzy. Focus on the edges and the centre is really quite fuzzy. Focus on the right or left hand third, all is acceptable. Then the madness starts: focus on the bottom third line, even though the subject at that point is much near, and that point is sharp, the centre is sharp, and the edges are perfectly reasonable...
So the general rule is: don't focus on the centre. More specifically, for shifts, focus on somewhere near to but not as far as the part of the planar subject which is furthest away from the nodal point.
A really good technique, and on my lens this guaranteed to give you one good fame, is to set your shift, set your aperture, then focus one on each of the following spots:
centre
half way to the top third line
the top third line
half way between the top third line and the top of the frame
No doubt time will refine my technique but blimey, this lens has a non-planar, non perfectly curved DOF which varies with aperture, shift and distance to subject.
Bracket. But at least it can be done: here's a crop from the extreme top of a frame with ten shift, at 100% - and I must say, I can live with this!
(focus was on the pale stone just under the base of the flagpole...)