A final layer or two of mat on the inside would actually be good for cosmetic reasons. Polyester resin shrinks as it cures, so the mat helps prevent the weave of the relatively course roving from printing through the final layer, and would require less fairing and sanding to prep for painting. Per a brochure from 1979 attached below, Potter used 4 layers of mat, more than anybody, between the gel coat and first layer of roving to prevent print-through.
Balsa core is a good choice for coring the inside of a hull above the waterline. It bonds well because resin soaks into it better than foam, plus it has at least 6 times more shear strength than most foams. For more info on core materials, check out this article by Dave Pascoe:
http://www.yachtsurvey.com/core_materials.htm