Im mostly chasing our Pa whitetails and even bear dont go far with holes in both lungs I go heavier when I head out west for elkMy every day arrow set up is 550gn all up.
We have water buffalo Here in Oz, they are genuine big game animal. I found 750gn works for me - have shot as high as 870gn. The heavier arrow didn’t really penetrate any better. I’ve always used 2 blade double and single beveled broadheads on big critters.
Ive seen vids of water buffalo I bet they are a tough animal