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Db, yes, my 400 II + 1.4xTC MkII can achieve similar IQ wide open.
Here is an example also shot with 1DX at f/5.6, similar minor cropping as in the previous duck portrait.
All of these lenses and combos are very capable as long as you stay at f/5.6. Once you get to f/8 combos, the performance deteriorates.
Peter, really love the detail, crisp n clear, those new gears of yours seem to be responding quite well in the hands of a master...what a beautiful pijun and image