@shinksma, @lowner - yes, you can bandaid a solution - so that it AF's in bright light- and it might work for stationary shots. You can state all day that it can- and OK But in the end, is it worth it ? Is it quick enough to catch a fast moving object? Just how much longer does it take to acquire a subject- let alone a moving subject? It's a band aid. Heck, you could even throw a 2X tc on it- and use liveview to autofocus. Now, if you were looking at BIF - IN FLIGHT - good luck with any of those methods. But then and again- maybe you will get lucky. But then, how much did you pay for your overall band-aided solution ? Is it a good plan to go in with- thinking that it's going to be perfectly autofocus ( if you tell me it focuses perfectly well, and leave it at that- I assume that it focuses as fast as it did before the TC, and that it does not hunt ! ) IMO, the OP is looking at BIF, as in post #1, as part of it. A better plan might be to use the 100-400L as is- and plan on purchasing a little longer lens later if he/she needs it.
Overall, you'd be better off attempting to get closer, or cropping the shot, than to put the band aided solution into play. Try it for stationary shots- yeah, sure. BIF ... No point reiterating it..
EOS 5D III, EOS 7D,EOS Rebel T4i, Canon 70-200 f/2.8 IS II, Canon 24-105L, Canon 18-135 IS STM, 1.4x TC III, 2.0x TC III, Σ 50mm f/1.4, Σ 17-50 OS, Σ 70-200 OS, Σ 50-500 OS, Σ 1.4x TC, Σ 2.0x TC, 580EXII(3), Canon SX-40, Canon S100
Fond memories: Rebel T1i, Canon 18-55 IS, Canon 55-250 IS, 18-135 IS (Given to a good home)...