^^^Note: Johnny is clearly stating that he is using the EVO intake purely for sound enhancement and not a performance enhancement for the car. One should always take the manufacturer's numbers with a grain of salt until you see it for yourself.
I would say, just my personal and not nearly expert opinion, that the most effective modifications are also some of the most expensive, and aren't going to net a ton of power. Jake Raby has developed some great improvements that both bulletproof the engine and increase its performance, but you're looking at a really high cost for it, at which point you're going to want to keep that car and get your money's worth. From what I see and read, Jake's done some incredible things, and for them you'll pay an equivalent price.
If you're not that attached to the car then for the same potential $20,000 you could be spending, you might as well combine that with a trade of your Boxster and knock out 1/2-2/3 of the cost on a new Corvette Grand Sport. It's no Porsche, but it's also not going to take much to pull an easy 50 more horses out of the thing when it already has more than double the Boxster's grunt. That was just for the sake of example.
What I didn't mention in my first post in this thread is where that plumbing change idea came from. Pedro's garage, where Pedro has come up with some great and pretty cost effective mods for the car, is another good place to look. Again, you're not going to get gobs of horsepower, maybe you'll get 20, but you'll get as much out of those horses that you do have as you possibly can when you start chipping away at Porsche's compromises. Have a look if you haven't already.
www.pedrosgarage.com
be forewarned, this site can really tax your computer's resources, so don't have too many things running at the same time when you go to it.