I am not an expert just a nose picker. But 450gr bullets are where the 458winmag comes to life. I loaded my 458wm with barnes ss and x bullets at 450gr. I got almost 2300fps before they looked a bit hot so I backed down to a bit over 2200fps.(loads werent compressed) I have talked to quite a few who get 2300fps with their 458wms. Mine is a bit slow I guess.
I did take both a cape buffalo and an elephant with mine it was very hot and she shot great. If you have to have 500grs IMHO punch it out to 458lott and be safe no doubts. If you want to shoot it as a 458wm try 450gr bullets.
Dean