I see that you didn't resist finallyIt seems redundant to have more than one telephoto zoom.
Not necessarily...I enjoy the flexibility of having the 70-200 2.8 for indoor use/sports, and having the option of the much smaller, lighter 70-300 for general outdoor photography because not only is it manageable for an entire day, it has 100mm extra reach...
I have the 70-200/2.8L IS II, also used for indoors/sports and sometimes portraits (when not using primes). I have the 100-400L for birds/wildlife (when I don't want to carry the 600 II). I have the 28-300L as a one lens solution that extends into the telezoom range.
I just ordered the 70-300L as a travel telezoom - lighter, but more importantly more compact than either the 70-200 II or 100-400, fits vertically in my photo backpacks.
As I have already mentioned I am interested in your impressions mostly regarding the opposite use of rings.
P.S It will fit nicely vertically on the bataflae 26L bag I just ordered