Im obsessed with my 2012 Denali, and its bone stock (except for LML DSP testing)....emissions intact. 116k miles and its had only ONE problem with the emissions components, a bad NOx sensor at ~90k miles I think.
It runs awesome, sooo quiet/smooth, I love the factory TCM tuning (compared to 2006-2010 6-speed), doesnt smell, and easily gets 19mpg highway. The brakes are amazing, the chassis is amazing, it handles great, the TCS/ESC (stabilitrak) calibration is bang on in ice/snow and not over-intrusive (hill start assist is pretty cool too), exhaust brake is great, onstar remote-link app is awesome...even the new 2012+ nav unit im pretty impressed with (nice graphics with animated menus, has some large/prominent city buildings rendered in 3D, text-to-speech for nav guidance), compared to the 2003-2011 nav unit that was lame early-2000's tech at best in terms of UI, graphics, and navigation capabilities. The USB power from the nav unit seems limited though, so if you have Pandora and google maps going on your phone, it charges very slowly.
It took a while to find a flawless-condition Denali like I wanted with nav, sunroof, cooled seats, and side-airbags....but I couldnt be happier with it. Best vehicle Ive ever owned. Ive had it for almost a year and I still love driving it, I honestly have zero desire to delete it...its been so many years and hundreds of thousands of miles of driving a loud/fast/built/smelly (big injectors)/creaky/quirky truck that I just got so tired of it. Yes, the LML throttle response needs help; it can sometimes feel really laggy, but nothing a pedal-to-torque map tweak wont fix.
I still have my LLY, and Ill never sell it just for sentimental reasons (and everything ive done to it would be an un-understandable headache for anyone else but me LOL)...every once in a while Ill drive my LLY just to charge the batteries and run the engine up to temp, and I do smile when I get back into it, but once I hop back into my LML, I forget about it again.
Admittedly, the GMT-900 seats arent QUITE as comfortable as -800 seats, the GMT-900 interior has its chintzy-ness (knock on wood, my dash hasnt cracked)....but those are nitpicky things.
If the emissions components ever start giving me troubles, I might re-think my decision to delete it, I dunno. And ill be CP3-swapping it when I can set some time aside. Definitely going to do everything I can to keep it as quiet as stock though, IF I ever delete it.
I have zero doubt that the 2017 L5P will be awesome. If I had the money, Id buy one the second they come out. For everyone whining about the "potential problems" with the emissions systems, big deal. It has a 100k mile warranty for that, so who cares???
Ben