I have two ways of reducing sound on my Boxster. One is a set of supertrapps that bolt on the exhaust and the other is a set of "Laguna pipes" that I built which are essentially angled pipes with a small insert and exhaust packing stuffed in them. The pipes didn't work well enough to pass sound - 92dB was the limit and I was hitting 94-95 dB. The supertrapps were more effective but I still have to lift when going by the sound station. My car without either is at about 98dB wide open.
Pipes installed - 3" diameter
Inside
Inside #2
Supertrapps