I've been shooting the D800E for a year. I've only had a 2-3 images with moire - and it was very hard to see in near out of focus areas on finely patterned subjects. I've shot about 8000 bird photos and only seen moire a couple of times. I have never seen it in event photos but I suspect that is a risk area.
I have seen moire a couple of times as a result of downsizing images - and its relatively easy to fix.
I would consider moire a non-issue for photographs. But if you shoot video, moire poses a much greater problem if it occurs, so the D800 is probably a better choice.
I think my conclusion is that yes, there is diffraction at f/8 and beyond. But DOF is much more important to the image - especially until you get to very large prints. And downsizing for the web or prints under 20 inches effectively overcomes the diffraction or significantly reduces its impact.
Given the resolution of the D800E, you can see differences in lens quality in that for the first time the camera is highly resolving the image to the point where lens differences can be seen with the naked eye at 100%. The practical side of that is 100% is far beyond any normal print. But I can see differences across my lenses - or even with two different models of the 70-200. My earlier cameras such as the D200, D300, and D7000 do not provide that level of resolution but prints look just fine.