Well, you might want to find out what all the 996 C4S had installed (headers and/or cats plus the PSE?).
There is a PSE option for the Boxster too or you can weld one up yourself - see Pedro's site for details: pedrosgarage.com/Site_3/Make_the_Pedro_Sport_Exhaust.html
The above will get you similar sound or you can go aftermkt w/ someone like Fabspeed (I have their exhaust on my 997S and it sounds great). That being said, only a 3.6L w/ PSE is going to sound like a 3.6L w/ PSE
Good luck