When I have to drill out an exhaust bolt/stud, I put in an stainless stud from an aircraft exhaust (you can pick up or order from any aircraft supplier). I've done many aircooled engines with them (no boxster yet) and are problem free when installed. The pics are 3/8" studs but there are different sizes. Make sure you get cad or silver plated nuts so the nuts don't rust(stainless nuts will gaul into the stud and is not recommended).
