From my personal experiences I decided I wanted something a little heavier than a .30-06 for zebra, wildebeest and eland sized African game.

However the 6.5x57 is very similar to a .270 and that most classic of African 'small' calibres the 6.5x54 M-S. All have killed game out of their size. If you want to and shoot it well, why not?

Haven't tried those bullets mentioned but they should have good penetration.