Anyone looking for the 200-400 + 1.4x range, should consider a 300mm f/2.8L IS II with 1.4x III and 2x III. I just got mine and that combo is super sharp and versatile. Will be selling my old 500mm f/4L IS and old 300mm f/2.8L IS now. If I need something longer than 600mm f/5.6, I have my 800mm f/5.6L IS.
The price of the 300mm f/2.8L IS II has also dropped recently to $6800.00 most places. Add the 2% bonus most places are giving ( Amazon, BH, Adorama ) and purchase with a rewards card ( Amazon give 3x points for items purchased with Amazon Chase card) and you can get a 2x III for nearly nothing. :-)
Well the big worry is will that 560mm be as clean as the old 500mm? If it is, then that old heavy 500 will be less missed than I thought.
And will I want to shoot at f5.6 for the duration of a day in shadows and late afternoons?....back to shooting at 400mm. Not that big of a deal. I do find when shooting with my 300, I am always wishing for a 400. 400 is useful for sports and larger wildlife. Quite a few times I had to run backwards so I could get far enough away for a full shot of a close Bobcat. Tree shooting would be great at 400 also.
Air shows, well 500mm was tight. I had myself wishing for a 400 a few times. It's too bad they could apply that inboard teleconverter to a 500mm. That would be a big seller. Sure, I prefer 500mm, but there are quite a few uses for 400. A revamp of the 400DO would have been great. I'm tired of my shoulders hurting! So the 200-400 still doesn't help that much.
Cams: Canon 1Dx, EOS M
Zooms: 17-40mm f/4L, 24-70mm f/2.8L II, 70-200mm f/2.8L II; Primes: Sigma 35mm f/1.4, 40mm f/2.8 STM, 50mm f/1.2L, 85mm f/1.2L II, 100mm f/2.8L, 135mm f/2L, 300mm f/2.8L IS II, 500mm II, 600mm f/4L II
Support: Gitzo GT4542LS/GT2830/G2258, RRS BH-55, BH-40, WH-200