Just for fun, I looked the 2.7, the 3.5 and the 5.0 up on fuelly.com

2.7 avg mpg: 18.3
3.5 avg mpg: 16.5
5.0 avg mpg: 16.3

As always, your mileage may vary..... but for .2mpg's, I think I'll forgo the complexities of the twin turbos....

Sic Semper Tyrannis