I took the 200-400 on a bird watching (spring warbler) trip. Thoughts, and 400 5.6 comparison too.

Thanks for taking the time to do this thorough review.

My main birding lenses are Nikon 200-500, Canon 100-400II, Canon 400DOII, Canon 600II.

I also own the 200-400.  It is my last choice for the type of birding you attempted with it.

I would recommend 400DOII with TCs if you are looking into this type of birding.  It is by far the best out of the 5 big lenses I own for your type of warbler test.  Unless you can really get close then the 100-400II (with or without TC) gives you much better magnification and is my pick for hummingbirds.  The Nikon D500/200-500 is also very good at warbler birding.

To address some of your points at least compared to my copy of the 200-400:

1) AF...I have not experienced the issues you had with AF...mine focuses super fast, accurate and in line with my other lenses mentioned above.  Granted I sold my 7D2 as that thing is useless for AF most of the time and use it now on 1DX, 1DX2 and 5D4.  That said, I did use it for a couple years on 7D2 and had no issues specific to that lens.  My 7D2 issues occurred with all lenses.

2) TC use.  I can't even tell IQ degradation with the internal TC engaged and I actually use mine a lot with external 1.4 and internal 1.4 without issue.  With both TCs I have had to do AFMA to get it critically sharp and it does a bit better stopped down a little bit.  But with only one TC it is tack sharp.

So I'm not sure what the main issue was with your copy.

Here are a few examples of what I can get out of 200-400 even with TCs on it....

I also included a few from 400DOII/2xTCIII which is my choice for warbler birding.

400DO/2xTCIII

200-400 with internal 1.4 at 560mm

200-400 with internal and external TCs at 784mm f/11

200-400 with internal 1.4TC