Nick,
don't know if this helps to narrow down on the root cause, but here is an image with the AFS 500mm/4 VR and D800E for your reference
Similar setup like you: Gitzo 3-series, Wimberly, D800E
f4, 1/320s, ISO 100, distance approx 40 meters
(
klick here for full size D800E image)
100% crop
rgds,
Andy