Another exhaust query
 

Anyone got some data on the OEM noise level design targets for the 75.
I'm sure I read somewhere here that MGR (or was it BMW) spent a lot of time arriving at the cabin environment noise/comfort levels and the exhaust design. I guess there were comparative external noise emission figures and some sort of supplier QC numbers to keep the exhaust system supply honest, so - I'm making enquiries to get a quote for a cat. back OEM-like SS system later this year. Are there any figures available for a 2500 KV6 that I can give my potential supplier to aim at matching?
Any idea how much the double skin back box contributed to the cabin noise (or was it there purely to stave off corrosion)? I'd guess the SS material gauge is going to be less than the OEM steel equivalent and presume that will result in a higher system resonance so does that mean the SS design will require more ribbing (stiffening).