Putting all that together, I guess I don't have a good option without the updated 100-400, Gah ...
... at first, I thought about waiting for the 100-400ii, too, but then people explained to me that it'll likely cost double of the mk1, so that's out of the question for me. *If* there ever is a mk2. From what I understand, maybe the Sigma 120-300/2.8 sounds the best choice for you - as always, it's a tradeoff between weight, cost & iq.
NA - I'll consider the existing 100-400 some more. OTOH, if they were to update it next year I know I'll be kicking myself
Concerning tc and f8 af: You can always get a Kenko tc (dgx) which somehow manages to fool the camera into af'ing though exif data etc. is correct. However, on my 60d the single point af really suffers w/ f8, and for multi-point af there are too usually few af points, I guess it'll be the same for your old Rebel.
Sure it'll cost more, but what does that Sigma cost, around $3,000? Not cheap and only 300mm. For a wildlife lens, especially since it may be with a FF body, I need 400mm at least. Also I'm sure they'll update it, it's too popular a lens to let it languish forever. If recent trends are any indication I'd expect it in the next few years, they've already updated the 70-200 2.8, the 24-70, 16-35, etc. If nothing else they want an excuse to jack the price up! Anyhow the 100-400 wins in the versatility department besides.
Kenko sounds good but like I said I'm sticking with Canon Brand I think.