Rolly,
You're right, 2-inch groups are plenty good enough for buffalo, since they're somewhat larger than the apparently squirrel-sized whitetails so many hunters shoot these days.
You might want to talk to your PH about the hippo hunting. Most are indeed killed in the water with brain shots, because it's the most practical way to do thejob, and if so a 300-grain Partition works just as well as a solid. Penetrating the brain doesn't require a solid, as it does on elephants, since bone around the brain isn't nearly as thick. Vast numbers of hippos have been killed with brain shots with "small-bore" rifles with expanding bullets.
Hunting hippo on land is a far more chancy method, since they're not often found out of water in daylight. I know this because of hunting hippo in the Selous Reserve of Tanzania a few years ago, one of the best places to TRY to get a hippo on land. I got one fleeting chance at a big bull at maybe 25-30 yards as he ran though thick brush, so chancy I decided not to shoot, but could have brain-shot many in the water. For dry-land hunting a solid is advised, because they're BIG, requiring far more penetration than a Cape buffalo. But even on land, the range certainly won't be long, so body shots 3-4" from point of aim at 100 yards won't matter.
