I'd do a 200 2.8L with a 2X. That way you've got 200 at 2.8, and 400 at 5.6. Get a 1.4 too if you want, and you've got basically a 200,300, and 400, at f/2.8, 4, and 5.6, respectively.
Sigma 120-300 is nice...expensive, though. An incredibly heavy lens comparatively, though. Don't know if you mind weight. It's really a tripod/monopod lens after the first few shots.





