FWIW a hot loaded 500 grain bullet in a .45-70 Ruger #1 is still 400'/sec slower than the same bullet in a .458 Win Mag. That's 1,750 ft-lb less energy. Further there's a lot of folks that aren't very excited about the .458 Win Mag either....hence the excitement over the .458 Lott.

Data is from the Hornady reloading manual.