Time will tell on the fuel economy thing. If you've got ideal cruising conditions, you'd like to think this turbo 4cyl would do very well. Certainly much better than 18mpg average.
You can consistently get 25mpg with the 5.3 on the highway if you keep it around 70mph. I don't know how any motor will do much better unless it's a hybrid setup. My best has been 33mpg over 50 miles. That's just happens to where I have it set at.