The new sabots leave little to no residue in the barrel. I have shot hundreds of sabots out of some rifles and never noticed a build up at all. I do occasionally take a brass bore brush, dip it in a good solvent like Montana X-treme cowboy solvent or Birchwood Casey bore scrubber and will work that brush through the barrel. That removes any plastic, copper, lead, and gunk that might build up. A few solvent patches and I am good for another couple hundred sabots.
As for powerbelts being soft lead.. you want to use soft lead with muzzleloaders. Powerbelt's must be effective or they would not sell so many of them. Some people report lack of blood trails. I believe a lot of that is shot placement. Any poorly placed bullet can leave a bad blood trail. Also many try to push them too hard. If you slow them down a little, some people get excellent results with them. But then a good sabot load is hard to beat. And normally cheaper to shoot.