I can't quite remember but in my 2004 F-150, I noticed a increase in gas mileage and power all the way up to 10,000+ miles. I gained about 2 mpg and gained some power. The same thing is happening with my 2006, slowly but surely.

 

I noticed it too. Especially on responsivness when taking off. Also saw it in my gas mileage since I keep detailed records on a spreadsheet.

 

I've had three of these new body style 5.4's and all three of them got more powerful and got better mileage as they loosened up around 10k miles. I dont abuse them when new but I do drive them normal to hard...75-80 on the highway and fairly quick takeoffs. Every one i broke in that way always felt stronger than the one you baby. In the last 17 years I've had over 100 new company cars and the one's you break in hard always run the best.