I've been shooting sabots in my Knight for amost 7 years now and have experienced no trouble with plastic fouling from sabots. When at the range I do run a spit patch between shots, but that is to clear out some of the powder fouling for consistent results - those patches won't even touch plastic. I've put up to 50 rounds thru the rifle in a day's time like this without any loss of accuracy - so obviously there was no "plastic" problem.
I will say that when I'm done shooting and am cleaning the rifle, I do make sure I clean for plastic as well as powder residue. Usually this just includes the scrubbing with a brush I normally do for powder residue, but when I am satisfied all the carbon is out of the bore I then will run a couple of patches of CVA's Sabot Shooters cleaner or acetone - either will remove any remaining hint of plastic.
From what I've experienced, heard, seen, etc, about the only possible problem one might have from plastic build up is from shooting max loads (150gr of powder) all the time. Quite frankly that kind of load is overkill and typically results in poor accuracy anyway. Keep your powder loads under 120gr (preferably 100 or less) and you will have no problem with plastic fouling.
As far as WHICH sabots to use, I've found that Hornady 300gr XTP Mags make a great practice bullet and their 300gr SSTs are super accurate and perform well on game. Like Rantingredneck said, I too have been disappointed by the powerbelts performance on game - I've had the aerotips punch right thru with zero expansion - even on heavy game like hogs, that ain't good.