Andy,
I was in the same situation in reference to a more sport sounding engine that is why I got the Supersprint exhaust (rear exhaust and tips) and the Evo Intake system. When I return back home I will make some wave files and email them to anyone who wants to here what this 986S should really sound like.
I am also waiting for someone in the USA to sell me the headers and cats as they are new items and last I checked it was 3 to 4 weeks wait before then arrive here in the USA. That was 2 weeks ago. Any provider from the
www.supersprint.com website should work as they all come from the same source in Italy. When I have found someone here that has them in stock I will let everyone know.
In regard to existing supersprint exhaust on my Boxster S I am very happy with the sound. Highway driving at 3500rpm +/- is more pleasing to me (louder over stock) but, no booming (back firing) has ever happened to me in my 986S and I drive like a mad man. I have never heard of this on the Porsche Boxster? However on my 1976 Lancia Scorpion which has a Race 4-1 header and fully race prepared engine it does backfire on deceleration only due to the large amount of advance timing on the intake cam. I will add that when you get down on the gas pedal (Evo intake + Supersprint exhaust) the sound is great all the way thru the rev range and is nothing compared to the stock 986S sound!
Ciao,