So the primes alone still seem to take the cake. How about F4 to F2.8? One would not think it's much of a difference. Does anyone think twice about it? or just make it up with a higher ISO?
Also pondering the use of a 1.4x with both. With the 2.8 it would yield an F4 at 560mm. Any resolution tradeoff between that combo and the 500mm f/4 outright? One would think but who knows.(Someone who has tried it and made the comparison).
Check the TDP ISO12233 charts. My sense is that the 400 II takes a bigger IQ hit with the 1.4xIII than the 500 II or 600 II. The new 500/600 + 1.4x seem equivalent to the MkI 600 and the 800, respectively, while the 400 II + 1.4x seems to fall short on IQ vs. the 500 MkI (and the bare 500 II is even sharper).
I think twice about it all the time. Not because of the light loss, but the loss of image quality. I know adding the 1.4 Extender will give me more reach and I know I'll have to pay for it when I pull the images up on the computer screen. Plus living here in Nevada with the dust, I dread each time I remove the camera from the lens/extender. And, the fumbling in the field changing the extender and the resultant lost shots.