I've always heard it this way:
Benefits of porting work by way of pressure. The higher pressure you're working with, the more pressure the porting has to use (without getting into all the details of porting in and of itself).
That being the case, shotguns run at a relatively low pressure (we're not talking comped nine major here). To be fair, I don't own a comped/ported shotgun (yet), but wonder if we're getting into that point on the curve they call diminishing returns.