Tom's right - bullet size is almost the most important aspect of cast shooting, providing the alloy is up to the task. .458's are very forgiving of alloys as well and even straight WW at Brinel 12 will shoot to around 2,000fps, sometimes faster, given a good lube. Most plain-based bullets of correct size will handle up to 1,700fps.
Ordinary beeswax/vaseline mix will lube WW bullets to 2,000fps with good results using slower powders. Lyman's newer moly lube (not the old graphite lube) and LBT Blue are good for higher speeds.

Gas Check bullets of appropriate hardness and with the correct lube will handle up to in excess of 3,000fps.

I have never hear of keyholes due to seating depth, either.

Like Tom - I have moulds form 300gr. to about 580gr. in around 50 gr. or closer increments. Every bullet weight shoots well from a variety of .458s.

With a large throat diameter, the actual bullet diameter might have to be considerably larger than .459 or even .460". Groove diameter is non critical - if it's the throat causing problems. I've not hear of problems with factory .458 before, though, and a .459" to .460" bullet should work fine.

What is the inside diameter of a fired case's neck? Ideally, with cast bullets fitted to the throat, the cast bullet should be just a push fit into a fired case. You can try things such as teflon tape or paper patching is that is necessary to get a large enough diameter bullet. Do not worry about a larger bulelt increasing pressure - it is low.