I almost get the impression that this is for a specific assignment. If so, the answer is simple: rent a 5DIII and a 600 f/4. It gives you the same field of view as a 400 on the 7D, and you will not, no way no how, get better image quality, period, full stop, end of story. You also won't get better AF performance or any of the rest (at least, not until the 1DX hits the market) with anything else.
Cheers,
b&