The new 400mm DO compared to 300/2.8 and 400/5.6 for sharpness

[neuroanatomist]

To me, the difference in the two lenses gets down to if you prefer 300 f/2.8 or 800 f/8.

Agreed. For me, since I already have 840mm f/5.6, 300mm f/2.8 is more appealing.