A couple of obstacles to overcome to get true hifi sound in a Boxster.
*High ambient noise floor (90db+ on the highway) so you will need a high SPL to get well above it and hear fine musical details.
* Stock Becker components run at odd impedance and some have MOST fiber optics which are incompatible with industry standard heads, amps and speakers. For best results, rip it all out and start from scratch.
* No room in the cabin for a quality, high SPL sub in a tuned cabinet.
If you throw a ton of money at it, you can get an excellent mobile boom box but it will never rival the audiophile quality sound you can get in a dead-quiet Lexus cabin. My solution was a lot simpler:
Amazon.com : Shure E2 Sound Isolating Earphones : Shure Earbuds : Electronics
It's not a perfect solution but it works for me on long hwy drives. Since I rarely listen to music with someone in the passenger seat, no one feels left out. YMMV