3.7 OEM exhaust diameter
 

I am thinking of getting some chrome trim to brighten up the back of the 8, but unsure of what diameter I should be getting to put on the tailpipes. I just think it will look better with chrome rather than the twin black pipes popping out the back.

or you could see if A8 parts have an S8 Back box which would keep it OEM and do the same thing for you ?
Youll need to check if it will fit the 3.7

You can brighten up the ends with emery paper, wire wool as the system is stainless anyway +++

Standard back box exhaust tips
 

So are the standard back boxes stainless steel under the black paint.???