It appears from my research that I can use, and would get good velocity from AA 2230 in the 458wm and 500 grainers. Any good reason not to use this?

Not according to Accurate Arms:

AA loads for .458WM



They have found other powders work better; but feel free to experiment.



George

.458 Win Mag � 450 X & SS � 72.5 Gr. � AA2230 � 215GM � * Gives 2,250 to 2,300 fps and fine accuracy.