I can say with absolute certainty and based on experience, the turbos are not "wide open" the whole time. In fact if in go about 40-50mph on flat land I can return almost 20mpg towing 7000 pounds. Turbos are used during acceleration and that's pretty much it. More so than the 3.5 I'm sure but who cares (it's built stronger than the 3.5).
Unless your to towing over 7500 pounds, the 2.7 is the better engine. Unless your doing it every day, but then why buy a half ton.