I am still having a hard time deciding on this lens. On one hand, I want it. Having a 100-400mm f4.5 sounds great for subject separation and when the light gets low. Add a TC 1.4 and you get a 140-560mm f6.3 that should be just as good as the 200-600mm f6.3. It will also be a bit lighter, have a bit bigger zoom range, and take front and rear filters. Add a TC 2x and you have a 200-800mm f9, which is hopefully as good as the 400-800mm at 800mm (sounds like we need a bit more testing here to know for sure) and will definitely be lighter and have a much bigger zoom range.
On the other hand, it's hard to say from the initial reviews if it's substantially better than the 200-600mm and 400-800mm. Adding a 4lb and 4k+ lens that doesn't at least fully replace my 200-600mm is a hard sell.
As for the f4.5 aperture, if this lens were a 7lb+ 8k+ f4 zoom, I would not be interested in it at all. So I actually see the f4.5 as an advantage in this case. The only thing that may have added interest to me is if they added an internal TC, even if it added like 1/2 pound to the weight and a few hundred to the cost, if it improved IQ vs an external TC it would have been a value add to me.