The best thing you can do to improve your iron sight shooting is to buy a 22lr with sights not too unlike your 458's open sights. For me, a Win M9422 is close enough to the sights on my double or 375H&H to suffice.
Lots of cheap and recoil free practice at any range, indoor or out, including "pistol" ranges. Ever tried 100 458's in a day? Even loaded way down, the cost will still get you.
