I have made a couple of single cavity molds in stainless steel for casting full bore 12 gauge slugs. Once casting temperature is reached, the lead takes forever to cool before I can drop the slug. To speed things up I plunge the (filled) mold in a bucket of water for a few seconds after every four or five fills. This works great, but you must be careful that every drop of water has evaporated before resuming casting (don't ask how I know! )

The only scenario where I could see the benefit of a stainless mold would be when casting very small boolits, where it sometimes can be a problem of keeping the mold temperature high enough.