So...the 2017 upgrade is more than a few things. The new engine puts out 445hp/910ft/lb vs my 397/765, it's also got a new Denso injection pump so it will hopefully not have the issues the CP4 has had. Everyone that's reported back after buying or test driving says it's a monster and has significantly better throttle response than the LML to boot. All that said, my truck is plenty capable to do everything I need it to, I have no regrets buying a 16 even though i new the new engine was coming. GM is getting better with their quality, but i'm still not too terribly interested in beta testing a new engine for them.
Only differences 2015 to 2016 are the remote lock tailgate, updated radio, and wireless charging for creature comforts and the digital assist steering for functional stuff. While I haven't personally, people have had issues with the new steering. If you believe the shit written on GM-T.com, it's a crisis...but we all know how that goes.
One thing I will say about chevy High country vs Denali....the 2016 Denali came with HID headlights, the high country came with halogen, and they SUCKED. between the headlights and the horrible red-clay "saddle" interior color, I gave up a better deal on the HC to get the Denali. (another bright side to the 2017s: the HC can be had with a black interior.
As far as 2014s, they were still GMT-900 platform. HD's launched after the 1500s yet again.