I have the 300 f2.8 IS II that I use for aviation photography. Most of the time - maybe 90% of the time - I use it with the 1.4xIII to obtain 420mm. With this combo I'm very satisfied with the IQ and AF performance.
But sometimes the planes fly far away or high enough that I wish for more than 420mm. With the 400 DO II and 1.4xIII I'll get 560mm.
So if (and that's a big if) the 400 DO II has IQ equal to or exceeding the 300 f2.8 II & 1.4xIII, and have equal AF performance, then the 400 could replace my 300.
Here's what I'd get...
400 and 560mm focal length instead of my 300 and 420mm.
A weight savings of about 1 lb. Even more if I don't use the tc. This many help when panning.
These two benefits might push me towards the 400 DO II.
Yes... but if you pair the 300 with a 2xTC mkIII you will also have outstanding IQ.... especially stopped down to f7.1 or so. What happens if you ever need 300mm instead of 400?