FWIW, I used brass shim stock on an old H&R shotgun about 40yrs ago, it hasn't seen a lot of use considering the years, but it still works fine. But for rifle barrel, I'd use steel, but the brass may work for a while, some members are even using the temp shim made of aluminum.
The last 2 barrels that I shimmed, I used material that's slightly thicker than needed and filed it to fit after the epoxy cured, getting the springy blade in the perfect position isn't as easy as one that is already formed to the pivot shape. Dunno about durability of the shim material, as I haven't shot a lot of rounds thru one to really test it yet, so that remains to be seen, but I would think the blades are at least of as good a matieral as the soft underlug, plus if you use enough epoxy to fill the void in the underlug up to the cross pin, there's more support surface with the shim in place than a barrel without a shim, so it should last longer for that reason.
Tim