Sweet! Looks like the length might be similar to the 70-200/4.
zim said:
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).