I have no doubt that some barrels definitely profit from lapping, but the way they are made today any change will likely be minimal and not any better than just firing. After all, lapping (by whatever method) is just speeding up the wearing that comes from firing the rifle. You get a smooth barrel quicker, but you also shorten the barrel life by the same amount.
Yes, I have read those claims that "my rifle wouldn't hit a barn until I used Joe's (or Pete's, or Mike's, or Jim's) lapping process, now I get quarter inch groups at 5000 yards". I take them with a salt block, a grain being too small.