I think small diameter smoothbore smoothrifles are going to be totally dependent upon the type of game that one has to hunt. As well as the time, and effort that a shooter is willing to spend in order to get REALLY good with shot, as opposed to patched balls.
There is a man over in England that posts on The Muzzleloading Forum, aka Britsmoothy. He is forbidden by law to shoot bore sized patched ball out of a rifled, or smooth bore. He shoots, among other guns, an Italian, .45 caliber, smoothrifle import that looks much like a DGW rifle. His loads are near 28 gauge in powder/shot proportions. His harvests of squirrels, rabbits, and pheasants are constant. Most of his shots are 35 yards, and closer. It is my belief that most Americans simply give short shrift to the use of shot in anything smaller than a 20 gauge. As you go down to the 24 gauge, and the 28 gauge, the numbers of people actually hunting year round with these bore sizes decreases radically. And people just dismiss the .50 & .45 caliber smoothbores altogether.
It is my belief that the small caliber smoothrifle's are far more capable than they are given credit for. After all, thousands of them existed well into the percussion era, and they couldn't have always been used with a patched ball.