My usual lenses are Zeiss 21 mm f 2.8, Zeiss 50 mm f 2 Macro, Zeiss 100 mm f 2 Macro, Nikkor 85 mm PC Micro and Nikkor 200 mm f 2. All of these show diffraction softening from f 6.3 onwards. The laws of physics say it shall be so, but lesser sensors and lenses may obscure the effect with their own characteristics. The effect is significant by f 8 and I have to really need the depth to use f 8. I would rather step back and crop more. I checked out the current Nikkor 35 mm f 1.4 and the Nikkor 85 mm f 1.4 for my partner on a second D 800E. My recommendation was f 7.1 as the minimum on the 35 mm, and on the 85 mm for preference not below f 4 to keep DOF low, she finds f 2.8 ideal for portraits. So this is how we use the lenses and cameras and are delighted.
Of course none of the above is true at ISO 3200 or handheld at 1/30 sec!
I have checked lesser lenses like the 17 - 35 f 2.8 zoom and the 24 - 70 mm f 2.8 zoom. They are not so good at f 5.6 so do not show noticeable deterioration by f 8. They also have focus shift that really is unacceptable on a D800.