Diffraction is only one aberration that contributes to lack of sharpness. And this gets worse with increasing stopping down. In contrast, other aberrations tend to be improved by stopping down. In practise, what this means is that most lenses have a sweet spot where other abberations have been reduced but degradation due to diffraction has not yet increased intolerably. So stopping down further does not reduce the other aberrations but increases degradation due to diffraction. This is the idea of a diffraction limited lens i.e., the other aberrations have been corrected to a sufficient degree that the only thing that contributes to image degradation is diffraction, as one stops down further and further.Where this sweet spot is for any lens depends on the design and construction of the lens - in other words, based upon how bad the other aberrations are. Some lenses may need to be stopped down considerably to bring the other aberrations down, while other lenses may be sufficiently well corrected that stopping down a couple of stops will be sufficient. Within a lens line, problems with centering and quality control can provide some variance in the corrections.

I'm not sure I understand what you mean by long lenses should hit optimum at smaller stops. In an ideal world, all lenses would be diffraction limited wide open. Then stopping down is only a function of DOF requirements i.e., when DOF does not dictate, one can operate at wider apertures and still get pictures limited only by the diffraction at that f stop.

I haven't seen any data pertaining to differences as a function of different shutters, due to thickness of the blades etc. I doubt there is much variance there.

Cheers, DJ