I think the .300 Magnum is a good choice for leopard. A 300 gr makes more sense for a big giraffe bull. I would bring the .375 as well. If you decide to bring only the .300 Magnum I would recommend using something like Swift A-frame or a nossler for the giraffe and step up to a 200 gr bullet. I am not familiar with PMC's XLC bullets.
I don't believe Namibia is the best place for bushpigs. Rather come to South Africa.