Here is the 2.990" COL for the 450-gr #13 (top), 400-gr #13 (middle),
and 3.220" COL for the 404-gr Shock Hammer (bottom) for the .458 B&M+:
Those are equivalent to 3.250" and 3.480" COLs in the .458 WM+,
using R-P brass in the former and Hornady or W-W or R-P brass in the latter.
Here is my guesstimate on how the shortening of barrel to 16.875" will affect MV for .458 B7M+ or .458 WM+,
Here is how the .458 B&M+ did in the Tupperware stock, when barrel was 19.75",
5 shots with 450-gr #13 at 50 yards, mean 5-yard velocity =2203 fps mean of 5 shots.
77.0 grains AA-2520, 3.015" COL, I pulled one:
Same specifics for the 420-gr FB/HP "Raptor", for three shots only, I pulled third shot so quit there,
2223 fps mean of 5 shots, however,
Very close to same POI for the 450-grainer and 420-grainer using same powder charge: