I'll try to answer that for you. His truck is a 2014 and didn't have magnetic ride suspension like the 2015's, so it wasn't an issue. So yes, there is an issue with leveling the 2015's with magnetic ride. Check out this thread and I think you'll find your answer.
http://www.gm-trucks.com/forums/topic/168618-leveling-kit-for-2015-sierra-denali-magnetic-ride/page-5?do=findComment&comment=1601164
Look on the last couple pages and there's a company who will level and adjust the sensors. Someone has also talked to a GM tech who claims he can re-calibrate the sensors once the level has been installed, although I don't think anyone on here has confirmed yet.
Hope that helps.