Have you fired wax slugs into water, gel, or newspaper? From what I've seen they break up into shot again only a couple inches into the target and cause little penetration. Firing them into trees I can normally get them to take off some bark but barely damage the wood past 1/2" deep. I'm not sure how they would handle the ribcage; they may break up early. This is with target load wax slugs. I have made wax slugs over heavier charges and most of the time they break up leaving the barrel. Some members here on CGN have had luck with hot glue slugs which are the same as wax slugs but made with hot glue instead of wax. They are on my list of things to test but I haven't gotten around to it just yet.
Personally for the price difference I would just go with factory slugs. It's only a few dollars more.