According to this:
http://en.wikipedia.org/wiki/Porsche_Boxster#550_Spyder_50th_Anniversary_Editio n
it used the m030 suspension. Harder springs and higher damping struts. With the 18" wheels, this car should be pretty stiff and bumpy over uneven surfaces, but great handling in the corners.
Maybe your tires have stiffer sidewalls than what were on the 550 that you drove.