My favourite lens used to be the 300 f2.8 VRII, just stupid sharp and great overall IQ and this was backed up by DXO Mark which had it as the equal 2nd sharpest lens by Nikon, the 200 f2 VR being the sharpest @ 34 P-Mpix and the old 400 f2.8 VR and 300 f2.8 VR one point behind at 33 P-Mpix - nothing in it really. However, this new 400 f2.8E FL VR is just in another league, it is as sharp at f2.8 as it is stopped down! Put a TC on it, any TC, and you can hardly see it affect the IQ, even wide open!! There has been controversy about the new test site "Lenscore"
http://www.lenscore.org/
which rated the 400 f2.8E FL VR as the sharpest long tele they have tested, but I can vouch for it, it is ridiculously sharp and detailed with great overall IQ. Not only that, but every other aspect is a large step up. AF speed is blindingly fast and accurate, the bokeh is excellent as is contrast and colour balance.
The lens bare is stupid sharp and fantastic overall IQ.
Add the 1.4x TCIII and it is sharper than the 500 f4 VR, even wide open!
Add th 1.7x TCII and it is sharper than the 500 f4 + 1.4x TCIII, even wide open!
Add the 2x TCIII and it is sharper than the 500 f4 + 1.4x TCIII, even wide open!
My 500 f4 VR is now up for sale.
Some samples I took yesterday on my D810 to try it out:
@ f3.5, ISO224
100% crop of above
With 1.4x TCIII @ f5.6, ISO1425
With 1.4x TCIII @ f4 WIDE OPEN!!, ISO141
100% crop of above:
With 1.4x TCIII @ f5, ISO283
crop of above:
With 2x TCIII f7.1, ISO1008
100% crop of above:
With 1.7x TCII f4.8 WIDE OPEN!!, ISO400
100% crop of above:
Bare @ f3.2, ISO64
crop of above:
Bare @ f3.5, ISO64
100% crop of above: