You can purchase a genuine "04" for under $15.00, no tax, free shipping from here:
Porsche 911 Boxster Cayman Expansion Tank Cap FEBI BILSTEIN New 99610644704 | eBay
I replace mine every year as part of preventative maintenance; I don't even test the existing one. I've heard of water pump failures because of old, faulty caps, so I don't even mess with them.
It may be an old wives tale, but then again, I haven't replaced a water pump yet.
Just sayin'...............
TO