My guess would be 2150 to 2200 fps out of the 16 1/2" barrel, and if the velocity is lower than that, changing to another powder should bring the velocity back up.

The suggestion about asking some oddball handgunners is good though, I have seen custom built bolt action pistols in .375 H&H and .458 Winchester, and have no doubt that some masochist has built one in .458 Lott.

Richard.