I posted this in the cast forum some time ago but thought it might apply to your question. These loads are on the extreme end of what I would call light in the 458 you have virtually no where to go but up from here (unless you go to a lighter bullet) To really get the "feel " of what the rifle can do you need to use another powder, but SR-4759 is great for light loads.
I have used IMR SR-4759 for reduced loads in the 458 WM with good results. No fillers are needed even at low loading densities (25 grains with a 520 grain Lyman), the loads are mild and can be accurate if the bullets are good. Of the bullets I have cast, the best ones would group into 1 1/2 or less at 100 yards with the 4759 loads. I have also used the 450 grain Lee bullet with similar loads of 4759 (28-32 grains) It groups about as well with even less recoil. All of my shooting has been from a Ruger # 1 and I think groups could be better if the bullets were better. I am still having trouble with mould fill-out. Make sure you get all of the copper out of your barrel before you start shooting the cast slugs for best results. The lighter 405 jacketed bullets made by speer and remington ( I dont have my speer manual handy) should also shoot well with light recoil with SR-4759.
enjoy
PS velocities with the above loads should be in the 1000 to 1200 range I think, this is w/out a chrono.
captainkirk
EDIT
from the speer #12 manual
26 to 30 grains of sr 4759 gives 1262 to 1445 fps with their 400 grain soft
in a ruger #1 24 inch bbl All of the other loads are in the 2000+ range and you will feel the difference. Several powders will get you there when you are ready for that, IMR-4320, 3031, 4895, 4198, and 4064. The 500 grain bullets and heavier will recoil much more than the 400s. 'course this is all relative, the win kicks less than the lott which is less than the 460 etc... enjoy
the straight wall case is no more difficult to work with than it looks, just don't bell the mouth too much...