The .454 plenty potent enough for grizz inside of 100 yds. if the shooter is well practiced with the rifle. Lots of folks tend to go with heavy magnums because they've been convinced that bear & moose are tough to kill.
A PH I've hunted with twice in Zimbabwe keeps a Freedom Arms .454 in the Land Rover for two reasons. #1 is the ease of carrying compared to his John Rigby .470 double when wandering off into the bushes for his morning constitutional. #2 is to deal with problem elephants that feel like messing with the Land Rover. While it's not a preferred way to deal with the situation, the couple of times it's escalated to that point, the result was a dead elephant.
Based on those results, I'm going to vote in favour of the .454 being an adequate bear defence round