Have shot targets with cast bullets in the .375 and deer, bear and range cattle with the very excellent LEE 402 grain {wheelweights} Hollow Point bullet in the .45-70 at 1640 fps which would make it a likely performance level for your .458.

Remember, HARD does not necessarily mean the elimination of leading. An undersized, hard bullet will lead like the devil. I want my cast bullets to be .001" or .002" over groove diameter. A softer bullet will upset some, fill the grooves and if lubed properly lead very little at all or none at all. You may find leading more annoying in cold weather than hot.

The LEE is actually just about .0005" under groove in my Marlin and I had to work up a load using a felt grease wad and card column to eliminate all leading in summer, with just a smidgen in winter.

If you can find that LEE HP {I believe it is discontinued} you will have the best .45 cast hunting bullet ever made for deer and black bear in my opinion. It acts just like a Nosler and sheds the front HP portion with the shank penetrating deeply. One of the range cattle i shot with it was injured by a car, raving mad and charged me hell-for-leather and that bullet penetrated 24" of head, neck vertebrae and shoulder muscle. It is a very impressive performer. I once shot a doe deer with it, she standing angling from me at about 40 yards and at the shot she flipped and when I walked up to her there was a piece of liver the size of a fingersteak laying on the ground next to her with an exit wound you could slide a pack of cigarettes through without getting it bloody, the bullet continuing on toward the next Zip Code. Another incident where that bullet proved itself was when I had to shoot thru a 4" thick fir bough to get to the chest of a treed bear. Bullet penetrated the bough, the bear's forearm, chest and exited out the critter's back. I do believe you will have a tough time recovering one of those things inside a game animal if you cast them from wheelweights and shoot them at 1640 fps.