You could have the old style coolant cap which Porsche updated, cause the old one released coolant at a lower pressure than desired. Check if you have the old one or not. Also, these guys are correct, if you do add water, make sure it is distilled water, and not plain water or mineral water, or else you'll have to flush the whole system.
Left is old coolant cap, right is new updated one (it's only like $25 bucks at dealer):
