Scott,
Being that the car is heavier than a 2WD Suburban due to the AWD system and bigger engine, you may not be able to get that much more. My wife's 2WD Escalade has a little over 13K miles on it, and she can't get any better than 13-14 MPG. And that's being really easy on the gas pedal.
My advice to you would be to do a K&N cold air intake system and a good dyno tune in order to get more power while getting better gas mileage. That is what I plan on doing to my wife's Escalade and I should be able to get 2-5 more MPG along with more power. My buddy picked up 4 MPG on his 2007 Escalade ESV with the K&N Aircharger and dyno tune. He also picked up close to 50AWDHP (all wheel drive horsepower) and the before and after drive confirmed the huge difference in power. Since you have AWD like my buddy does, you will need to visit a tuner that has an AWD dyno so you can get it tuned. The 6.2L engine is a really good motor, but it lags off the bottom and needs a good tune. GM's calibration is terrible and there is a huge room for improvement. The Escalade is a great platform to start with, but with a good tune, you can get amazing results.
SG
