Some good advise here so far.
We all get emotionally attached to these cars and we all want to go faster. Aftermarket companies know this and constantly dangle add ons promising performance gains like shiny lures in a trout pond. It's pretty hard not to bite.
Before going shopping take the emotion out for a minute and get an engineering perspective: What do I really want this car to do? The more clearly and honestly you answer this question the more likely you will make the right changes to the car and be satisfied with the result. You will avoid some pretty big pitfalls and your wallet will thank you. If 217 hp just doesn't do it for you are you sure 240 will really ring your bell? Are you willing to spend $2-4k to find out?
Most Boxster enthusiasts usually fall into one of these three groups:
1. I want faster lap times at the track and a streetable car every day.(This is my quest and I still have some work to do.)
2. Modding cars is fun and I enjoy tinkering and testing almost as much as driving. I don't get in much track time and I do it for joy not for specific results.
3. I want to build a fire breathing street racer and out drag race Vettes, hot rodded Hondas and Subies.
All of these are valid reasons to modify. Which one are you? Do you want the best bang for your buck or is money no object?
If you fall in with #1's the mods that give the fastest lap times per $$ are: weight reduction, performance driver training, tires, and suspension, in that order . Lots of guys regularly turn top ten times at the track with a stock Boxster motor against some formidable drivers and cars with twice the power. Somebody has figured out how to make these things go fast. Highlander, Insite and JohnV come to mind. Why not you? If you are already in the top ten every week and the only guys outrunning you are Scott Pruett and Andy Lally it's time for a motor transplant
and a chat with Roger Penske.
If you find yourself with #2's the sky is the limit. edevlin probably has one of the most highly tuned and modded 2.7's around. He spent a bundle($6k??), it took nearly a year to get it all sorted out, and it probably still would not pass smog in Calif. but he loves his car and it sounds like it works very well. He enjoyed the process and I think he is very happy with the result. There are lots of others like him here.
If you fall in with #3 it can be done, just bring a dump truck full of money. You could probably build 3 or 4 hot rod Hondas or Fords that run a 10 sec. 1/4 mile for the same cost as one 10 sec. Boxster. It probably won't be very comfortable on the street, won't pass Calif. smog, will have lots of little issues, but it will be very fast for a while. I don't think we have any 10 sec. cars on this forum yet but a few are headed that way.
Whatever path you choose don't forget to just knock the top down and take it for a spirited mountain cruise once in a while. Nothing does that better than a Boxster.