Thinking of doing the same thing. Took the car into Cartunes a specialty audio install place near me that normally has 'pimped out' rides that they work on. Last time I was in they were working on a CL600 and an H2 that could have blown your ear drums three miles away.
In order to get in the door speakers, they need to order a special kit (as there is no space in the door). If you look at the Boxsters that have factory installed door speakers you will notice that the panel juts out a bit to allow for the speakers to be installed.
The company they looked at that made the aftermarket kit was Cascade, if I remember it correctly but cannot find them online. The kit will allow you to install either 2 6.5s or a 6.5 and a tweet. Or you could get the factory panels and go from there.
