EF 400mm f/5.6L USM image quality

That list was the stupidest thing Canon ever did.  They simply wanted to sell more lenses, which likely worked.  That list was debunked years ago.

I used the 300/4 L and 70-200/4 L (both non-IS)on the 5DII, and the 300 was significantly better than the zoom with an EF 1.4x.  I now use the same zoom and a 300/2.8 (non-IS) on the 5DSR and the zoom and EF 1.4x is almost as good as the 300/2.8.  That tells me that the 300/4 lives up to it's name as an extremely sharp lens.  The 400/5.6 L is of the same era as all those lenses and will be extremely good on the 80D and even the 90D, and going forward for years to come.

When people say that a lens is not good enough for high MP cameras they do not know.  I have been using several lenses from the 70's and 80's on the 5DSR and the results are superb.  The Nikon 400/2.8 AI-S and Canon FD 800/5.6 L are both sharper than the 300/4 L I referred to earlier, which still puts them at an elite level.  The Nikon 8/2.8 AI-S is almost as sharp, but with less compression, than the EF 8-15/4 L that I tested.  The Canon FD 55/1.2 SSC Aspherical and Canon FD 50/3.5 Macro are the sharpest lenses that I own and are superb on the 5DSR.  Those that tested the 55 versus EF 50/1.2 L said it was a virtual tie, due to convenience of AF of the 50, with an edge in IQ going to the 50 year old lens.

In my opinion you can ignore that list, and put the 400/5.6 L to good use on any camera you like.  You can also do further comparisons on the digital picture website which will support this.