There are two broad but distinct aspects of this issue which I think it helps to be clear about. One is the theoretical, diffration limited resolution of a lens in terms of the sensor pixel count that comprises the image, ie. does the additional pixel count of the D800 serve any useful resolution purpose when the lens is used at F22, for example?
And the other aspect is the practical significance of such loss of resolution due to diffraction, at particular print sizes.
The first theoretical aspect is a guide or hint as to what to expect. The second aspect you really should dertmine for yourselves through making your own comparisons. Experiment a bit. The LiveView feature makes this process so much easier than it used to be because you can now be certain that your focus is exact.
I'll always remember the surprise I experienced when I bought a Canon 5D and compared resolution of the same scene shot at both F8 and F16 using the Canon 24-105/F4 zoom at the same focal length. The difference in sharpness in the plane of focus was very marginal at 100% on the monitor, representative of a really huge print. However, the differences in sharpness away from the plane of focus was very much greater in the F16 shot.
In other words, in order to achieve that significant increase in DoF that F16 provides, compared with F8, the sacrifice in sharpness at the plane of focus was relatively insignificant. I made other comparisons ranging from F4 to F22, and found that F22 seemed to be the aperture at which there was a clearly noticeable fall-off in resolution, compared with F8, F11 and F16. I try to avoid using F22.
Years later when I bought a Canon 50D, which is effectively a cropped version of a 38mp full-frame DSLR, I compared resolution at various F/stops with my 10mp 40D, equivalent to a cropped 25.6mp full-frame. I wanted to know if F16 would provide any resolution increase with the 50D.
Surprisingly it did. Resolution at F16 with a good quality prime lens, the Canon 50/1.4, was approximately equal to resolution on the 40D at F11, at the plane of focus. Of course, away from the plane of focus, the 50D at F16 provided significantly sharper results than the 40D at F11; in other words the 50D provided greater DoF without any sacrifice in sharpness at the plane of focus. That was a worthwhile improvement of the 50D sensor which many uninformed people on the internet claimed would serve no purpose at apertures beyond F8 and certainly F11. However, at F22, it was clear that the 50D provided no additional benefit of a practical nature. That aperture of F22 was effectively the cut-off point.
My advice to you guys is, get out your cameras, set up your tripods and take some test images for comparison purposes so that you can get a feel for the magnitude of any differences observed at various F stops.