Don: very interesting and informative posts. You seem to be one of the few people who consider diffraction in relation to sensor pit size. I know that Thom Hogan - a respected American commentator and Nikon user - mentions that the D200 hits the diffraction limit at about F13 and the D2x at about F11 (if my memory serves me). It is curious that many people comment that the advantage of the DX sensor is the removal of the nasty image edges where aberrations are worst. And yet, as you clearly show, a D2x can outresolve a lens even at the centre. Another aspect is that a lens usually improves as you stop it down, reaching a peak somewhere between F5.6 and F11. So if you use the lens wide open, the DX sensor may well crop soft edges, but it will also produce a softer image on axis compared to the so-called full frame camera.
Anyway, my own rule of thumb is to use a lens at F8 or F11 if possible, and F16 only if I need extreme DOF. For macro work the slight loss of sharpness in the plane of focus at F16 is countered by the increased sharpness over a wide depth.
I suppose if I had the uber-glass that some forum users own, then I would not bother with anything so vulgar as stopping down.
Leif