My statement merely recognizes the fact the photosites of the D3 are larger, hence support a large circle of confusion, hence support smaller apertures (and greater DoF) to realize better performance in the presence of diffraction effects all other things being equal ... this is wrong?
Yes, that's wrong. It's like saying, an airliner flight takes hours but a bicycle ride to the grocery takes just five minutes so bicycles are faster than airliners.
Are you saying that the D3 and D3x will have identical performance relative to diffraction effects for the same f-number?
No, I don't say that. Instead I'm saying the same what Bernard already said above (see post #9): the D3X will have better performance due to higher sensor resolution.
By the way, that shot I posted was at f/20 at 2x mag... so f/20 (1+m) ... well you do the math ... looks sharp to me ...
It looks sharp because you don't have a sharper image of the same subject to compare it to. And it clearly is sharp enough to make a pleasing image. However at, say, f/11, central sharpness at the plane of focus would have been even better. Of course at the same time, sharpness before and behind the plane of focus would have been worse ... and thus the overall impression of sharpness would have been lower. So for this subject, it was the correct decision to stop down, even when it did compromise maximum sharpness due to diffraction.
And the point is: Had it been taken with a D3X and the same lens at f/20, sharpness would be no less (slightly better actually which however wouldn't show up in such a small web image---printed at poster size, it would).