The corrosion of metals in sulfuric acid is complex and minor differences in impurities, velocity, or concentration may significantly impact corrosion rates. When austenitic stainless steels like 316L corrode they usually do so by means of a form of localized corrosion like pitting, stress corrosion cracking, intergranular attack, etc. I am not familiar with the specifics of corrosive attack of austenitic stainless steel by sulfuric acid, but if localized corrosion occurs, a corrosion allowance (increasing the metal thickness) will offer no benefit. Corrosion allowances are only helpful when uniform corrosion can occur. You might consider running a corrosion test in the laboratory to simulate the exact conditions of your process to gain additional insight and determine what types of corrosion may be seen in actual service.