I do not yet have D800 (waiting for my order to be filled), but wanted to remind you about diffraction which kicks in after f/8 on D800. So past f/8 you should expect some loss of sharpness if you're looking at your image in full size. Also, from what I read, the mirror might move quite strong, so that too can add to loss of sharpness. Did you try f/8 and mirror lock-up?
You can also refer to Nikons technical guide for D800: