It needs only a 1.4xTC popped on to give 400-800mm as it has an internal 1.4xTC.
I can believe that, especially as the digital picture found better IQ at 560mm with an external 1.4xTCIII than with the internal. https://www.the-digital-picture.com...meraComp=979&SampleComp=0&FLIComp=6&APIComp=2I wouldn’t recommend that approach. I got better results both IQ and AF with the 2x than without the internal 1.4x engaged than the internal 1.4x + external 1.4X TC III. You may found different result if you have used the wins.
I understand that 200-400 can be a great safari and sports lens but for me who shoot birds it would be a heavy burden. My 400DOII weighs 2.1Kg and even my 500II weighs less than the 200-400 and is a stop brighter.
How does that help?