Excellent comments here - especially from Alan and Len.
Macro specialist Mike Moats has demonstrated the same facts about diffraction vs. DOF. With macro subjects and other situations where you have a tiny DOF and want more, DOF trumps diffraction. You can easily shoot at f/22 or even f/32 if the situation warrants. Yes there is an impact of diffraction - but DOF trumps diffraction.
Now a related issue with the FX cameras is while diffraction shows up at apertures a stop or so lower, you may need more DOF. FX needs about another stop compared to DX. And the high resolution of the D800/D800E means when you view an image at 100% you may see softness in slightly out of focus areas that were not previously observed.