Velocity loss with a shorter barrel is nowhere near the big deal with a 458 that it would be with a smaller magnum caliber like a 7mm Rem Mag.
Why? Because while the two cartridges have more or less the same case capacity, they have very different bore volumes. The 458 bore is roughly the same diameter as the inside of the cartridge case, so a 20" barrel represents 10 times the volume of the inside of the 2" long cartridge interior. The 7mm mag barrel has less than 4 times the volume of the case. Save your worrying about that last couple of inches of barrel for a 7mm Mag beanfield rifle, not a 458 WM DG rifle.
Bottom line: With a 458, pick the barrel length that handles best and don't worry about the tiny amount of extra velocity a longer barrel might bring.
