I've also been looking at these. I'm attaching an article that's worth reading, if you haven't already.
http://www.pmletter.com/Topper.html.
From what I've learned so far, it's a pretty well designed/built bolt-on. Of course, I've also heard that any supercharger makes your engine sound like there's a vacuum cleaner sitting beside it.
I've contacted TPC and they got a quote of $5,700, + $1,000 more installed. I believe this included the ECU upgrade required. To get the full 300 hp, I think you also need to add a good header and exhaust kit, and upgrade air intake. I've also found their supercharger offered through 3rd parties, including this one for $5,495. Good picture on this site.
Recently I found a turbo kit that boasts 330 hp for a 2.5 liter Boxster; includes headers, intercooler, cylinder head machining, etc. (A wopping 420hp for a 3.2l -- WOW!) This sounds pretty impressive, but I'd guess the longevity of your engine will be a bit less than with the supercharger. I bet the turbo sounds awsome. However, I'm no expert. Hopefully someone out there with more know-how than I also replies to your posting.
Here's a link to the turbo kit company: TTP.
http://www.ttp-sportscar.com/boxster.htm. I don't know much about this company, but from what I've seen, TCP is pretty reputable.
It'll probably be 2-3 yrs before I can afford to put another $10K--you'll want to upgrade the brakes as well--into my 99 Boxster, so I'll be very interested to find out what you do and how it works out for you.
Best of luck.
Ted