You shoot a 500+ grain bullet with 150 grains of powder.. you better make sure your teeth are glued in. That would have a heck of a recoil. As for, do heavier bullets need less powder? That is sometimes true, but your rifle is the one to judge that. If you really want a nice heavy lead bullet in .458 I would try the 405 grain LEE flat point. I just got done casting some up, and they ran a little heavy as I remember. They look almost identical to a Remington 405 flat point but instead of the copper jacket are pure lead.

I think Sabotloader has some good loads for that caliber worked up. But if you need pure lead for legal hunting reasons, that's where I would look. I shoot them 405's out of a lot of inlines and usually 90 grains of powder. Anything down range of that bullet is in for a world of hurt.