I'd say "this could cause speciation". This mutation's rarity makes for an astronomically slim chance of it occuring naturally if enough sinistral (lefty) snails meet and create a sustaining population (and also not die out through inbreeding). I don't think there is a case of a pair of chirally opposite species. Maybe this once happened with Amphidromus inversus but if that's the case, the two species have mutated since to be able to successfully mate both homo- and heterochirally. Now, a balanced population exists and hetero mating is more common.

However, as humans come into the picture and can find mirrored snails and purposefully put them together, and breed them in safety into a large population (collecting newly found mutants worldwide and adding them into the gene pool to avoid inbreeding), speciation can indeed happen. The resulting mirrored snail can fulfil the same environmental niches as the original species while having an almost completely separate gene pool (only the mirror mutants of each species can cross-breed - and if my above theory is correct, the advantages of tapping into a new gene pool may have helped dextral/sinistral then-subspecies of Amphidromus inversus eventually acquire unique breedability).