Problem with sd1 Merrill and 10-20 4-5.6

Well I can give you a rough idea of how I use the 8-16, both in the past on the SD1 and also on the Quattro.

You would think the contrast detect focusing on the Quattro would work better, but a consistent flaw I've seen is that the 8-16mm likes to focus past infinity.  So basically if I';m using AF I often double-check to make sure it's not gone past infinity - if not, the focus usually has been fine.

If it has gone past infinity, or if I'm just shooting a subject more than a few feet away, what I will often do is move the focus to a point just a bit before infinity.  That usually leaves most of the foreground and the background pretty sharp even at f/5.6.

I find it very interesting that the small shifts in focus  would impact the periphery to the degree you noticed.  I have often though the focal plan of a very wide lens is probably distorted so you might actually need to alter focus a little to keep things off to the side fairly sharp.  As you say the effect is kind of random as to small shifts though, so it's not like you can use micro focus adjustment to account for it... I think the problem is that seen through a very wide lens, the features you are trying to focus on are too small to register reliably either for phase or contrast focus, so it's a bit jumpy.  If you had wider or larger marks than a ruler, it may be more reliable.  I suppose if you were really serious you could take a focus target with large markings out in the field to focus on...