In the summer of 2014, I had the opportunity to test the 400mm f/2.8 for an entire day, thanks to Nikon Switzerland who was present at Air14, a very big air show which lasted over 10 days. At that time, the only modern long focal length I had was the Nikkor 28-300mm f/3.5-5.6. The 400mm is quality wise probably the best lens I experienced: however, its bulk, cost and area of deployment (most racing, planes) made that I decided to go for the AF-S Nikkor 300mm f/2.8G IF-ED VR II; together with the TC-20E AF-S III. Now, about 2 years later, I took about 8'000 pictures in 300mm and 3'000 pictures in 600mm. The quality of the 300mm is really outstanding; and remains excellent with the TC-20E III. One of the best purchases I made with a reasonable cost / performance ratio.
Hereunder a couple of pictures, the third one with full aperture, which is my normal working aperture with this lens; it is mounted on a RRS baseplate, quick release clamp and most of the time on a carbon monopod. Standard modus operandi is "VR ON".
(1) 300mm f/5.6 1/320s ISO 110
(2) 600mm f/8 1/400s ISO 110
(3) 300mm f/2.8 1/4000s ISO 100
(4) setup D5 / 300mm / RRS / Vectronix
(5) crop 100% (1200x1200px) 300mm f/8