One of my buds is a 30-year brown bear guide on Kodiak. For all his adult life he's done all his personal hunting with flinters, so I thought he might have useful insights.
He's never popped a brownie with a muzzleloader or seen it done, though he's been on hand for more bear kills than most of us manage with deer. His personal favorite flinter is his 54. Asked about a 58 for Kodiaks, he said "something bigger" with wheelweight balls. For interior grizzly he speculated that a 58 with wheelweight balls would be just fine with a good hit.
He added that you want your backup shooter to be someone you have good reason to trust to stand his ground and shoot well and fast under pressure. His pick for backup, based on a whole lot of holes in bears, is a 375 H&H with 270 grain Remington softpoints. He won't let his assistants carry anything else. Second best is a 416 of some sort.
He didn't differentiate between coastal brown bears and interior grizzlies when it comes to backup. You want to do a lot of damage in a hurry, no matter whether the bear is pointing at you or away from you, and no matter how far away it is. Even if it's running straight away, "shoot hair and keep doing it till it stops moving." You just don't want to let it get out of sight, especially in brush.