I have used a wad when I shoot Triple Seven powder and a patched roundball. I find that the wad protects the patch from being blow apart or burned. This allows the patch to do its job. So would a wad under the ball help.. all you can do is try some. If you have a hollow hole punch you can knock some out of milk carton cardboard. It works very good. A 1/2 or 9/16th works very well. You can also use wool felt and make your own.

I even took a 1/2 inch copper pipe, cut a chunk off, and sharpened the edges of the one end.. and used that as a hollow hole punch. It did work but not real well. Set a board over the top of the pipe before you hit it with a hammer.

With the kind of accuracy you are currently achieving, I doubt if a wad would improve anything. But again.. muzzleloaders learn by trial and error. Good luck.