Don't waste your money on a strut brace. Some feel they are worth the money because they spent money on them. I can tell you that I have put them on lots of different cars that don't handle anywhere near as good as a boxster and they didn't make any difference at all. On a car as sharp as a boxster the difference would be even smaller than a regular car that would benefit from tuing the suspension over the soft stock setup.
Anybody who says it really did make their car faster is likely right, but only because their wallet is lighter reducing the overall weight of the car lol.
http://www.pca.org/tech/tech_qa_question.asp?id={088B74B4-0905-45BD-BFC8-6103F8F577BE}
http://www.renntech.org/forums/index.php?showtopic=9424