I think the only use a --edit-- has in a slow twist barrel is if the shooter wanted to try firing a smaller diameter roundball from the existing bore. There are many variables that can allow or prevent this. It would be a matter of the shooter experimenting with it for themselves.
Somewhere I have some --edit-- that allow the shooter to use a .32 projectile in a .36 bore, .36 in a .45, and .40 in a .45. If you use a round ball as the projectile, then you have no worries about rate of twist. All that being said, I have yet to try them for myself.
It's hard to beat a well fit PRB. Especially in a barrel designed solely for PRB's.