Over the past 50 years I've pattern tested more than two dozen individual muzzleloading barrels, mostly 12 gauge plus a few 20's. That by no means makes me an expert but from that experience I can say I have never found ANY advantage to plastic shotcup wads. I know it is common for gun writers to claim 20% improved pattern density by just using plastic shotcups. I'd bet dollars to horse apples those gun writers have never fired even one test to confirm that claim. They just heard it somewhere, it sounded reasonable so they repeated it, that's what gun writers do.
Many years ago V.M.Starr, a noted blackpowder shooter and gunsmith who actually tested his claims, said you cannot get better patterns from a muzzleloading shotgun than that obtained with just two hard card wads over the powder and one thin card over the shot. My own far less extensive experience confirms that statement. Fiber filler wads, often called cushion wads, are commonly used to carry lubricant to soften fouling and make successive loading easier but they offer no pattern improvement and often will spoil the pattern, producing doughnut patterns open in the center, just what you don't want.
There is no way around it and no shortcuts, if you want to know what works best in your own individual gun you just HAVE to shoot patterns on paper and count up those itty-bitty holes. I would suggest you start with equal volumes of powder and shot with 11 gauge overpowder cards between. If you look on it as fun it will be, if you look on it as a tedious chore, better stick to modern guns, modern shotshells are terrific.
