I still have the OE muffler on my Denali with the butterfly valve open. Opening the valve helped add more sound but does require me to drive in M7 since the AFM is now more audible. This will be no big deal after my BB tune. Question- from watching videos is doesn't appear the Magnaflow really changes much of anything. I know the muffler is only $125, but labor will without doubt be 1-2x that to have it installed. Those of you that have done the change, was it worth $300 +/-? *UPDATE- quote I was given is $275 for muffler install and oil change. No clue if that is fair being how every oil change until now has been free and the shop doing the install is ballparking the estimate.