Sometimes it is just a waste, other times it may be more accurate.
Sabot is a very general term, all it means is that there is something around the projectile allowing a sub bore diameter projectile to be fired. That then seperates from the projectile some point during flight, or sometimes upon leaving the muzzle, it all depends on the design.
Since most are designed with the intention of being used in a rifled bore the spin of a rifled bore is often expected for thier design to function as well as intended.
However there is many sabot designs specificly made for a smoothbore as well. Many of the kinetic energy projectiles of main tank (M1 Abrams tanks use smoothbore) guns are just such designs.
A sabot design which uses fins to impart its own stabilization or which is designed to use air resistance to shed from the projectile needs no spin imparted by a barrel to work flawlessly.
So it is not a simple answer, because there is a very wide variety of sabot designs.
Most sabots designed for use in shotguns are designed for use in a rifled bore.
Some of those work well in a smoothbore, some do not. Some take longer to seperate from the projectile from a smoothbore than designed.
Some make the projectile very accurate out of a smoothbore and some will be little different than a foster slug out of a smoothbore, while costing you much more per projectile.
You will find some people who report they get excellent results from some sabots even out of a smoothbore at much greater distances than a foster slug.
So it is not a simple flat answer. It depends on multiple things, the design of the round, the design of the sabot etc
