So you are saying that jerking your pump action off like a hormone ravaged schoolboy and hoping to get one "good/lucky" shot is good? My point is that you maybe will get only one shot off, and the salvo capability will be useless. Unless, like Why Not? suggested a while back, one is being charged by a herd of grizzlies.
The .458 will actually have a muzzle energy of ~4450 ft-lbs and 142.86 lb-ft/s of momentum.
The top Remington 12g load gives 3006 ft-lbs and 109.87 lbs-ft/s of momentum from its 437gr (1oz) slug. This is load SP12MRS.
The top Remington Sabot load gives 3086ft-lbs and 104.50 lbs-ft/s of momentum from a 385gr slug.
That is an advantage of around 40% in energy and 30% in momentum for the .458 WM. The .458 is so superior that they aren't even comparable.
I doubt there is a single guide in Alaska using a 12g to go into the alder thickets after a wounded bear. I would bet that the .375 H&H, .416 Rem Mag and .458 Win Mag would cover the majority of guides' choices. I would do as the pros do. They probably get more action in one season than you and I will in our entire lifetime.