I have a 2012 Denali and have done a few basic things. I have an Airaid JR. intake, Corsa catback, removed the 3rd cat, and a Black Bear Tune. I did the tune last and it by far gave the biggest performance gain and mileage gain. I honestly cannot say I noticed much of a mileage or performance gain from other stuff (still glad I have them though).
Before you bother doing any mods for more power run premium gas. After trying both regular and premium I can definitely tell the power difference on premium.
I knew I wanted more V8 sound from the car. My first mod was Airaid JR. it added a bit of V8 rumble but was hard to tell much else for power and sound and mileage.
Second was Corsa sport exhaust. Loved it but was a bit too quiet still so I removed the 3rd cat. That definitely woke the system up to my liking. Still no drone and basically disappears into the background when you don't want it but sounds great when you punch it. Again, didn't notice a big power or mileage bump.
I've got a detailed review and before and after video clips here (scroll down a few posts to see the Corsa and then Corsa without 3rd cat).
http://www.tahoeyukonforum.com/forum/showthread.php?t=66402
Finally, I just finished the Black Bear Tune. The car feels the way it should have from the factory now. Basically just less lethargic. It's finally willing to downshift without a huge throttle stab. It pulls a lot stronger now. The shifts are quick and precise without being harsh or jerky. It idles smoother as well. I left AFM enabled but it is a lot smoother- it doesnt cause the vibrations through the floorboards like it used to.
Post tune, my mileage is up quite a bit too. I seeing about 20 on the freeway where I was seeing 17 before. I think the tweaks Justin made to the AFM system play a part here.
Stock I could barely tell when the engine was running in 4 cylinder mode. After I added the intake and exhaust I could feel and hear when the truck kicked AFM on. It wasn't bad but it was audible and I could feel the vibrations. Since Justin tweaked it I can still hear when AFM kicks on but it is a ton smoother when it does. Much less vibration and a bit less sound.
so that is my $0.02.
---------- Post added at 11:17 AM ---------- Previous post was at 11:12 AM ----------
Ok, I really like the stock lope/rumble at idle, and I don't mind a little drone, I like to know the 6.2 is there, I just don't want it obnoxious. Which is why I may start with an intake first...that made a nice difference on my 6.0
Just a heads up the stock exhaust is a bit louder on the early models. When they added AFM the exhaust got a lot quieter. I much prefer the sound of the early exhaust to what mine was stock.
Does your truck have two or three factors cats? Mine had 3. I wonder of that was part of the sound dofference in the early vehicles.