You don't need open sights UNTIL the buffalo runs away with a hole in him and then you do....... When he comes for you out of nowhere, you REALLY need 'em.
Consider the cost of an African safari compared to the cost of a set of open sights and it's a no brainer....... just go and have a set fitted.
I'd highly recommend the red fibre optic foresight on the business end. They're a little fragile but work an absolute treat in the thick stuff.