Those smaller manufacturers would probably give you a recomendation of power level- they offer a couple three different clicks up and down.
Your problem is that these loadings will probably not be in Africa.
Federal was one of the first to offer a 300gr at 1800fps. One of the others does too, but whether it is Rem or Win or PMC I dont know.
There isn't anything that a 300 gr .458 projectile at 1800+ fps can't do in the lower 48. I'd like more for Grizzly, but I'm paranoid.
IT doesn't make sense to get the 308 or 3006 chambering. You can get a semi auto in these.
Someone needs to make a affordable strong double.
You might consider a Ruger number 1. One shot, but, in say, 416 Rigby, a 'best' effort.
munk