Canon's own MTF charts show the 300mm f/2.8 II + 1.4xTC III (bottom) to be significantly better than the 400 DO (Top).
Just checked Bryan's TDP comparison: the 300 at 420mm is distinctly sharper
Well, I did too, and it's not "distinctly sharper" except in the extreme full frame corners. I would say "somewhat noticeable" on the full frame borders, and "not much different in the center".
What about Canon's own MTFs? Have they got it wrong showing the distinct superiority of the 300 @ 420mm?
Yes, yes they got it wrong, because they just did, ok? They can't draw their own charts...Let's not get silly Alan. Look...I believe I originally said the 300 + 1.4iii combo was "not that far ahead" of the bare 400 DO, or something to that effect. Their MTF chart shows the 135L as even softer than the 400 DO, but the one I own, is not soft at all. Why would that be?
In any case, if you must defend the 300 ii as if it is your child and I'm attacking it or something, go ahead. But just know that's what you're doing. Frankly if your 300 were your girlfriend it would have already whooped your ass for cheating on it with the Tamron!! How do you like them apples?
It doesn't change what shows on Bryan's test...in the center of the image, anyway. Like I said, by the borders, and especially corners, yes the 400 DO is quite bad, wide open (and even closed down a tad). But it's a DO, designed perhaps 20 years ago. It's not a terrible lens given this, especially if it can be bought for $1000 or more, less than the new 300 f/2.8 ii (either new, used, or refurb...but I'll admit not many are out there). And the 400 is a pound lighter. The 300 is the more versatile lens overall, and performs better overall...but you pay for that. It doesn't mean the 400 DO is not worth owning and using, if someone wants lighter weight and doesn't mind the slower AF and lower performing IS...yet still wants f/4 at 400mm.
Canon build both lenses at the present time, and they have not stopped production of the 400, that I'm aware of. Are you calling them fools for doing so? I think you are...and they (yes the whole company) want you to step outside and discuss it in the street!