I contacted Supersprint in August and they sent me a quote for my 320d GT.
They don't state anything about any gains, if there even is any gains, but it looks cool
As my car is a lease, I'm not sure I'll be changing the exhaust. I've installed the PPK though, but that's another thing all together.
http://www.supersprint.com/en/IT/pro...093E280BA.aspx