300 f2.8 IS L II plus 2x Mk.III better than 100-400 f4.5/5.6 IS L?

With the 1.4x you'll get 560 mm at f/8.0 - stopping it down one step to get the utmost IQ out of this combination gives you f/11 - I'd this call unacceptable slow - not even talking about the 2x extender

Whereas the 300 f/2.8 II can be used almost flawlessly with the 2x extender III wide open at f/5.6. (which is one of the main reasons to buy a fast lens at the beginning)

Now I understand why some photographers need ISO 3200 and above at daylight already

My learning from handheld shots at 600 mm is that you need at least 1/1000 second to freeze motion and to be on the safe side not to have any part of the image blurred. With f/11 you need a very bright day to get that at ISO 100 with f/11 - not commenting on the DOF either - which would be my next sometimes (unacceptable) creative flaw on this combination

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