Not exactly bike related, or directly relevant, but a few years ago I was a tech inspector for the Legends race cars, which ran the 1200cc engines, with a Micron can.
One race meet we picked up a different exhaust note on one of the cars, so pulled the exhaust apart to compare with a brand new item. The new one had what I believe to be some form of fibre glass matting, whereas the 'revised' version was packed with wire wool.
The owner, and modifier of this exhaust is also the owner of a well known aftermarket stainless steel exhaust company, so I guess he had some idea of what he was doing...