Guys I know who shoot smooth bores really, really good swear by it! I've been in matches and seen 'em shoot too.
As but one example, a .672 ball, roughened up, and having not one, not two, but three separate coats of Alox lubricant. I suspect roughing up the ball brings it up to about .677 give or take, for a very tight fitting ball. I usually just don't have the time to roll coffee cans full of lead balls around to nick them up, and I don't have a pair of rasp files either... But I will say that the scores show folks who employ the method are doing many things right, and this arcane ritual may be one of 'em?!
Plenty of practice is my main hunch...
Good luck and good shooting to you.