. . . I was wrong. Yes. And in this case, I am delighted to admit it.
The 600mm f/11 does capture a greater level of detail with the 1.4X extender attached. Yesterday I was looking at files on C1, forgetting that DXO renders raw files with more sharpness (although C1 does a much better .jpg export).
Out with the combination under moderate overcast skies, the results without harsh light and with less heat shimmer were noticeably better. A hood is on the way from the Land of Cheap Accessories, which should make things even better.
Today’s photos show that the lens is good with birds (more details in the raw files than what can be displayed here) even with the extender attached. For a diffractive optics lens, it handles backlight well. and AF is quick enough to capture BIF.
It’s almost enough to make one ditch the idea of wating for the 100-500mm and go for the 800mm—but not quite. Hopefully, it’ll prove to be worth the wait.