Here is what NOT to do if you are looking to improve fuel economy: get rid of the rear muffler.
Leave it alone. Its there to create back pressure and improve fuel economy and doesn't do much muffling. I would say replacing just the rear muffler with a straight through Borla cost me 2mpg with no noticeable difference in performance. The car sounds right, though. I just could not stand the total silence of the stock exhaust..... And that ugly pipe in the back.