The Boxster is not really a car you should be buying if you like to performance mod. That was the whole point of my buying a Porsche, it would be set up from day one. All my mods have for the most part been visual. The way the BoxsterS motor is designed you will have to spend an oscene amount of money to get anything near 50 horses added on. imho a 300hp car is not ALL THAT quicker than a 250-260 HP car. In my local autocross chapter I have matched or beaten cars with 50HP advantages, M3's, Mustangs, Standard Carreras, STI's, etc. And I'm not even that great a driver.
And again, its a roadster. These cars are meant to be light, responsive and sharp in steering and handling, all while getting a tan.... they aren't inteded to be one dimensional horse power beasts. And With Boxster you get the best braking of any convertible in this price range. Maintenance has been far below what I was afraid of. I've put 35K miles on my BoxsterS and unexpected repairs have been limited to one throttle body replacement for less than $300.
here's a good link by a forum member on Boxster maintenance and stuff.
I used it to do my 15K and 30K mile servicing.
http://mike.focke.googlepages.com/mikesporscheboxsterwebpages