The reliable and friendly Swedish experts have measured the MTFs of the 100-400mm II, including the 1.4xTC to go with their earlier tests on the 300mm f/2.8 (+1.4x and 2xTCs), Sigma 150-600mm S and Tamron 150-600.
http://www.objektivtest.se/tester/canon-ef-100-400-mm-f45-56-l-is-ii-usm-test/
On full frame: at 300mm, the 100-400 II at f.5.6 is very close to the Canon prime and beats the Sigma and Tamron, with similar results at 420mm for the 300 + 1.4xTC and 400mm on the other two. The 100-400 II + 1.4xTC at 560mm beats the Tamron at 600mm and is as good as as the Canon prime + 2xTC and the Sigma.
On APS-C, at 300mm the 100-400 II is as good as the 300/f2.8 and better than the others, repeated at 400mm (420mm on the 300 +1.4x TC). The 100-400mm at 560 easily beats all the others at 600mm.
The results bear out my own experience, especially that the 100-400 II + 1.4xTC is sharper than the 300mm II + 2xTC on the 7DII, which initially surprised me. However, on FF he 300mm II + 2xTC is still king.
http://www.objektivtest.se/tester/canon-ef-100-400-mm-f45-56-l-is-ii-usm-test/
On full frame: at 300mm, the 100-400 II at f.5.6 is very close to the Canon prime and beats the Sigma and Tamron, with similar results at 420mm for the 300 + 1.4xTC and 400mm on the other two. The 100-400 II + 1.4xTC at 560mm beats the Tamron at 600mm and is as good as as the Canon prime + 2xTC and the Sigma.
On APS-C, at 300mm the 100-400 II is as good as the 300/f2.8 and better than the others, repeated at 400mm (420mm on the 300 +1.4x TC). The 100-400mm at 560 easily beats all the others at 600mm.
The results bear out my own experience, especially that the 100-400 II + 1.4xTC is sharper than the 300mm II + 2xTC on the 7DII, which initially surprised me. However, on FF he 300mm II + 2xTC is still king.