Well this is my first post, I am joining to contribute and not be the guy who reads everything and never posts anymore .
I had a '14 Sierra Crew with the 4.3 that I traded in with assistance from GM, so a somewhat 'buy-back' just a few weeks back now.
Prior to moving into a new truck I had a couple requests regarding the issue and I was willing to keep the GMC:
1) Continue to warranty the shutter after the 3yr/36k warranty: I was told this was not an option, the repair usually only lasted a few weeks anyways.
2) Disable AFM: Also told this was not an option. Between myself and the fence post if I had to live with it I was going to either have it tuned out, or buy the Range AFM delete after the warranty period.
When my trade in assistance was approved it was strictly implied that my replacement truck needs to have a V8. They know it is an issue with the 4.3.
My advice is to contact GM directly about the issue, it is well known IMO. I did it with little expectation and actually had a choice to make within a few weeks, I did have to be persistent on the phone call but everything else went smooth.
The beauty is you get a case # from GM and can reference that on your CSI surveys after service...really helps the process move along.