trond,
I've had my Bisimoto Pulse Chamber for roughly a year (you can check my original post if you like, it's here somewhere) that documented the process and results.
First, the volume. Yes, it's on the louder side as is, and the decibel level may not be for everyone. I installed the sound arrestor (which Bisi does make available) and the volume goes WAY down with no loss in performance. Have you inquired about it? Before you dismiss this exhaust due to volume level, install the sound arrestor and enjoy your radio with the windows down.
Second, performance. We all love our butt dynos but they are no match to a proper dyno session. I've had my 3.2 dyno'd twice at Bisi's shop and we saw an increase in HP and torque on both sessions. Have you checked the article about Bisi's pulse chamber on European Car? That is my car in the article. You can see the sound arrestor installed as well.
Exhausting Porsche Power - Bisimoto Pulse Wave
And third, I have never met someone who wholeheartedly stands behind his product and will take the time to explain the science behind the gains, like Bisi. He and his team are a class act, and I have nothing but praise for their product and services.