We have a 2014 5.3l Yukon-XL. The lie-o-meter usually says 13.x MPG average with semi-urban driving but I've never checked it by hand. We live rural but head into/through town frequently.
I too was worried about the horrors of AFM and it can be annoying a bit sometimes when driving. Around maybe 70-80k miles I noticed oil consumption went up (enough to need to add a full quart about half way into an oil change interval). Though I must admit I didn't really keep good track of how much it used prior. The next thing was a check engine light and misfire codes. That really got me concerned though I read online that most of the AFM issues were early models with a different valve cover design. I've also read that driving it hard occasionally is better than constant light use. That's not much of an issue for me.
When I pulled the plug on #7 imagine my surprise when I found half of the ceramic electrode missing! I put a new plug in and the misfire code didn't come back. We also had one O2 sensor go bad about then. The oil use is still there but not enough to be concerned about. Other than a broken seat handle and rear hatch switch (both less than $20 each), we've had no other issues with just over 100k miles on the clock.
I think it has the 3.08's. Whatever it is, it runs barely above 1000 rpm at 55. My only complaint is the aggressive upshifting makes it a turd at pulling out into traffic. Right when you need the power, it grabs second and bogs for a bit. Matting the pedal or turning on tow/haul fixes it. I just click tow/haul when I need to pull across or onto a busy road.
