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Seeing that you're doing a lot of "animal in motion" shots, the 7d2 would certainly be a huge improvement. But I applaud you for even trying with the 600d's system, with my 60d (same af system) I know how limiting it is.
I can't select the iso as exactly as I'd like. With the 400 5.6 I often want to shoot between 400 and 800 and I can't.
Just don't - intermediate iso stops are just digitally over- or underexposed versions of the base iso. It can be argued that due to Canon's specific low iso read noise problem, some 160x iso stops are a very tiny bit better, but really nothing to write home about or upgrade for. My advice: simply shoot raw with full stops and worry about the image content.
At 800 iso I have very little dynamic range to play with, so if the bird is black and white I'm in trouble if the exposure isn't perfect.
A 7d2 won't fix your b/w problem at this iso range, nor will any other crop camera of any other brand. The only available option with current tech really is to get a full frame, then you can try to squeeze the last bits out of the shadows w/o running into shadow noise or banding. It's a worst case scenario just like wedding dresses, but with animals you cannot use controlled lighting like with people.
I only shoot in manual, and having a bit more customizability might be good. Certainly having a couple of definable presets would be very useful.
Use Magic Lantern - available for your 600d, but not for the 7d2 (and no, it'll be not ready shortly after release, if ever).
I sometimes wonder if something in the middle would be useful! 100 2.8L?
I don't really know about your specific requirements, but the 100L is always a safe bet - great iq and the most fun lens since there's no minimal focal distance. The af is a bit slower though, it's a macro lens after all.