Well, If everyone is running the subs under old cruiser rules for shipping interdiction then having surface guns makes sense - a shell is a lot less expensive than a gun, and if you have to surface to check the ship for war material anyway, then it makes sense to have the guns.
Except that you don't need nine 8" guns to go commerce raiding. A pair of 5" guns would be more than enough to sink an unarmored, unarmed merchant ship. Once you factor in the cost of designing a submarine to mount some of these guns and their turrets I doubt you'd ever save any money over just torpedoing everything. Never mind the engineering challenges of trying to make something like that submersible.
There's a reason cruiser submarines fell out of favor so quickly; everyone realized they were a really, really stupid idea.