If I can reasonably hand hold it, I'd buy a 400-800
that's the only reason stopping me from looking seriously at the Sigma 200-500 f/2.8 and strapping an extender on it. The 200-400 extender will be close enough in practice though.
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, 7D2, EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 10-18, 15-85, MP-E 65, Zeiss 50/2 macro, Sigma 50A, 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye