If the 987 model Boxster shares 30% of parts with the 997 model 911, then how the hell can the Boxster be only 15%?

BTW, you must be dreaming about who makes parts, Porsche doesn't make 100% or even 80% of the parts that go into an old or new Porsche. The drive belts are 3rd party, the AC are third party and the rotors are 3rd party. The suspension is made by Bilstein and HECK, the damn transmission is Aisin (Japanese!).