I have fired a lot of shotgun shells, all since the 1960's with plastic wads, and have never had a problem with plastic fouling, either with factory shells or reloads in fixed choke guns. I have seen plastic fouling in barrels with screw in chokes, though.
As for tapered bores, could this have been done in an attempt to increase velocity? I know that target rifle makers lapped their bores to a taper, and there was some experimentation with taper bores in the military around the time of WWI.