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The results from your 5D2 after cropping will be very similar to what others have obtained using the 400/5.6 on a 20D or 30D - crop cams with the same pixel density as your FF 5D2. There are tons of pictures out there using this combo (just search pbase or your other favorite photo sharing site) so you can see whether the result is "close enough" to the subject.
Personally my bird-stalking skills pretty much suck and I found that I was often still focal length starved using a 500 + 1.4x on a 450D (1.5 times the pixel density of the 20D/30D/5D2). I feel I've taken a small step backward in bird photography now that I have a 5D2. It handles cropping extraordinarily well, but it requires cropping to an exceptional degree - so on balance it's similar to slightly worse than my 450D for small birds far away.
Of course the 5D2 is much better for landscape, macro and larger/closer critter wildlife photography so I'm still pretty happy with the change. Just recognize that bird photography is one of those disciplines that takes the least advantage of a FF sensor (unless you are a lot better than I am at getting close).
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