I don't think the D800 is at fault here. It looks like the camera has focused on the branches in front of your subject. Difficult shot with the branches and light background.Using a converter on your 300 f2.8 may well hobble it a bit. There will be some loss of quality with any converter, some more than others.
Visit my Nikonians gallery.
Copyright © Nikonians 2000, 2013
All Rights Reserved
Nikonians®, NikoScope® and NikoniansAcademy™ are trademarks owned by Nikonians.org.
Nikon®, Nikonos® and Nikkor® are registered trademarks of Nikon Corporation.