I have the 400 DO. It's not really "light" but it's much much less of a beast than the 400/2.8. I really like it - it feels like it hits a sweet spot for me in the weight/light gathering ability/quality trade off and the quality is indeed excellent. But there's an argument that a 70-200/2.8 plus a 2x TC is very close in quality these days, gets you more flexibility and is a bit less unwieldy for a loss of only half a stop in light gathering ability at 400mm.

Ooh. I'd love to try that lens. I have both the 70-200 2.8L II and the 100-400 II and I love them both. That 70-200 is one of my all time zoom faves but the 100-400 is not too far behind it.

Yes. I have the 70-200/2.8 as well and had the 100-400 mk 1 years ago. The 100-400 mk I isn't a patch on either quality-wise but I gather the mk 2 is significantly better so not sure how it compares.

Where the 400DO perhaps really shines is paired with a 1.4x TC - it's much more handleable than a 600/4 and paired with a decent body gets results as good as bodies 10 years ago would have got with the 600/4.

I have both a ~1999 Mk 1 and 2018 Mk 2 100-400. The Mk 2 is waaay better in almost every way: focus accuracy, no push-pull, better bokeh (marginally) better contrast… just better results with less effort. Also a lot more expensive.

I have three different TCs which I got from a friend who switched to Sony. I haven't really used them much because they seem to drastically change the optic behavior in ways I don't like. Maybe I should give them another chance as they never come out of their cases.