Hodgdon has some loading data using the 400 gr Woodleigh.

See:
http://www.hodgdon.com/data/rifle/405win.php

Just going by the Hodgdon data, the ol' .405 won't quite match the 450/400 ballistics but it comes close. Personally I wouldn't pick the .405 Winchester for Buff (either Cape or Water Buff). For African plains game in the bushveld, I think it would probably work fine. (Full disclosure: I've never hunted any type of Buff.)

-Bob F.