Just today stood next to a person using a nikon with a 80-400 lens. Seemed my crop factor meant that he did not have more reach (but maybe better quality, given that my setup is getting a little dated). I therefore think that the 100-400 is going to be enough to compete with some longer setups. It is not going to compete with the sigma 300-800 one of the other guys was lugging - but if that is your dream, then I believe it is already available for m4/3.
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Temporarily living in Tennessee