The way I read it, Stinger S, Stinger M and Stinger L are rolled separately (also buffs separately). But multiple Stinger S - such as Gluttony Guns - would add up to a maximum of 100% proc chance. I mean, if you got 7 of them (assuming that the current theory on skill level scaling is correct).
Anyway. Often when you start getting closer to 100%, the more powerful something gets... but I don't think that applies to Crits. It makes them more reliable sure, but I don't think that matters here.