eg3,
A couple of months ago I talked to the gent, see below link, who runs AR Designs. He was looking for a X6 to develop down pipes for. I think he mentioned 3" diameter. No one makes them but there are a couple of companies out there that make them for 335i's. They would be free but the car has to spend 5 days in Denver to get it done. I'm on East coast. I'd hoped to be able to help but it would be a 9 day trip for me. Perhaps there's someone closer?
http://www.ardesign.info/