http://www.consumerreports.org/cro/magazine-archive/2011/april/cars/best-and-worst/breaking-distance/index.htm
Not sure how your BMW 550i can brake quicker than the Boxster when it's the best out of all vehicles tested by Consumer Reports. I know this is a 2011 report but Boxster has consistently beat the chart for quite a while.
Since the boxster have very good brakes stock you shouldn't need to improve on it unless there's something wrong with your boxster's brakes.