My 2013 ecoboost Screw gets about 13 combined city/highway running 35's. No other mods. I'm expecting about the same or less from the new Raptors honestly. Granted, I live in Colorado where I do 80 on the highway and the altitude does have a pretty large effect on how the vehicle runs.
I'll sh*t a golden egg if the new truck gets better than 15 combined in real-world driving. If they are squeezing extra power out of the motor, it's most likely due to running a higher boost pressure which means higher fuel consumption for a given throttle input compared to the non-tuned 3.5's. Also means you're having to run premium octane fuel which increases the cost further.