The 405 grain Remington JSP has a harder core than the 400 grain Speer FNSP and is actually just as well suited to 1600+ fps.
Caliber, Cartridge, Mass, Bullet Type, Impact Velocity, Expanded Diameter, Depth of Penetration, Retained Weight
.458", 45-70, 400 gr, Speer FN, 1758 fps, 0.950 in, 11 in, 364 gr / 91 %
.458", 45-70, 405 gr, Remington SP, 2000 fps, 0.87 in, 13.8 in, 390 gr / 96 %
Source: Jamison and Strandberg