We have the ZL1 front guards also. They have only been on the car for 2 weeks (its a new 2017) but when installing them I did notice the tape was not pressed well against the flap. I installed them after letting the tape adhere with 75lbs of plate steel sitting on them overnight prior to...
Our 2017 PP GT according the dash display has averaged 18.7 and 18.4 the first two tanks of mixed driving. There were a few "spirited" romps on those tanks. So EPA ratings aren't too far off it would seem.
We just purchased a new 2017. I was a bit surprised how difficult it was for my wife to find a OW PP car. And then after a fair search, they were extremely willing to deal price wise.