Sweet! Looks like the length might be similar to the 70-200/4.
I think performance is a gimme, the price is what will dictate when I'd upgrade from the 70-300L (unless I can convince myself that both lenses can live together )
I bought the 70-300L mainly for it's compact size for travel (chose it over the 70-200/4 IS for the shorter length, more a concern to me than weight). I could see having both the 70-300L and 100-400L II in a kit. Perhaps not mine (I already sold my 100-400, wasn't really using it after getting the 600 II).
If this new 100-400 takes TCs and only a minimal IQ hit from the 1.4x (similar to the 70-200 II), I'd consider getting it as a more portable birding lens (size precludes the 150-600 3rd party zooms for me, that big I'll just take the 600 II).
Wasn't really considering my photographic requirements more how to keep them apart so wife doesn't see both in the same room at the same time
That's a good point about the TC
I do love the 70-300 when I'm on a 'two lens trip' though usually with a single wide prime
So one for airshows, sport and a bit of casual birding the other for general travel, holidays etc........ OK OK see I'm getting there!