I use the Sigma 70-200 f2.8 (not the current one but its predecessor in 2004) with a 1.4x TC and the results are excellent. Since the Sigma is f2.8 throughout the one stop penalty of the TC doesn't really matter. I've also used the lens with a Kenko 2x TC but the results aren't so good.

An alternative (and don't laugh) is to get a used Panasonic FZ-50. This has a Leica-designed 35-420mm f2.8-3.7 lens with IS. I've used this on Safari because it's much much lighter than a DSLR with a good telephoto lens. I also picked up an Olympus 1.7x teleconverter for Ł40 that extends the range to > 700mm but it's very difficult to use that combination with an electronic viewfinder.