The fact is that there has been quite a number of tests reported on this issue, and I don't remember any that showed a significant reduction in performance. The only assumption I can make is that they work well through some mechanism. I suspect that petal expansion is the answer, as I don't think the shotcups would expand to this amount. A telling test would to be test shotcup wads vs felt/fiber wads and check the pressure. I am guessing that fiber wad loads will perform much worse than the shotcups.
Also, look at some of the work being done recently with wadless shells (using only buffer products either below the shot or mixed into it). This is work by a reputable person in the industry and would seem to show that the whole wad issue is way over stated.
Also, I think inserts could have a lot of use in old doubes. When wads and hulls (say 10 gauge) are not available, they make shooting a gun possible. I think the biggest market being missed is the short shell vintage market. If the inserts stepped down from 2-1/2" chambers to 2-3/4" reduced gauge hulls, there would be a much large market.
