I found some published coefficients for teflon on steel. They showed .04 for dry condition. Polyethylene HDPE is what I believe is most of the wads are today. It's pretty slippery stuff and self releases from molds, which is why it's so popular for so many products. Tensile runs around 15 ksi. Some of the really tough steel type wads are nylon or other and can be very high tensile strength. Kinda gives a swag, but Miller's right; only an appropriate test would give the answer to any degree of accuracy.

But the notion that acceleration of the payload would not roughly pace with pressure due to differences in wad friction, when comparing significantly different pressures in loads otherwise the same, has little supporting it.