Nice choice!
Take advantage of the OnStar app and use it to remote start, unlock and other stuff.
A few guys around here are getting 21mpg on the highway with theirs. I think MS3DALE said that the last trip down to NC from Pa. his buddy's 6.2 got 21mpg but only 19mpg going back up to Pa. Still not bad.
Yeah, I went through the OnStar "let's try to upsell you on a bunch of stuff" process today at work. 6 months of OnStar and 3 months / 3 GB of 4G wifi hotspot coverage in the truck. I'll probably use some of that this weekend, just to see how well it works.
So far, the DIC is showing about 19 or so, running back & forth to work. Hopefully, I can actually get out of town for a few days and see how she does on the road.
EDIT : Oh, yeah! I'm not real impressed with their Android App. The keyboard doesn't popup when it should and when I "make" the keyboard popup, the box I'm trying to type in doesn't slide up.. I remote started it while I was walking out of work, but the app said remote start failed.

Hopefully, it'll get better with a few updates!