I'm sorry, but I just have to ask: if you wanted 300 hp, why in the world did you buy a 17 year old Boxster?

I dyno-ed at 288WHP on a Boxster S built to 4.0L by FSI. Getting ready to put a GT3 throttle body on, a 987 intake and a custom plenum. Then retune by FSI... I think this will get me close to 300WHP; I'm hoping to be over 300WHP.

It was NOT cheap to get to this point.

I think the best bang for your buck is the IPD plenum. It wont get you to 300Hp but its better than an intake or exhaust as far as power goes. Combining those three you can have good gains, and adding a chip too. Exhaust and intake help with sound but I feel like intake makes you lose power in these cars. I say enjoy the car, who cares about 300hp. Save up for a 3.8.