#2 1/4" or 5/16"?? Not looking for depth capability here so much as corrosion allowance and extra weight gain. NR-2 at this point, has 108 cu feet of submersible volume. More weight in structure is less carried in lead but, what do I give up in terms of workability with that added 1/16th??
Make the hull thickness one of your last weight budget items -- just put whatever weight is left over from other items into the hull. It's more useful than carrying around ballast, and even if you don't go deep you'll have a greater safety margin.