I suppose no one keeps a .458 around just for following up leopards but it seems to me pretty hard to imagine a better combination than one loaded with 300 or 350 grain bullets. For that matter, a lever gun in .45-70 or .444 Marlin might be pretty effective, too. Houndsman friend of mine took a trouble-making 180 lb mountain lion a few years ago and frankly, facing a leopard of that size I don't think there is any caliber or load I'd consider "too much"!