Not much speculation as the TTV6 has been in F150 for some time. Real world reports are the same mileage at a 5.0, and worse if the foot is mashed. It has proven to be durable to date. This will be motor amp'd a bit to 380hp and mid 400s in torque for 2015 version.
Point is, a direct injected and modernized version of the 5.0 is a less complicated, no heavier, and higher mileage alternative to the TTV6. And lower cost to produce and maintain.
Today with the Feds ridiculous low requirements for "emissions" including CO2, current flavor of the day designs lean towards turbos. But this is a red herring for meeting the standards.
I own a GDI TAU V8 version of the Genesis. That motor is the best I have ever had in a car. 32mpg plus at under 80 mph, amazing smoothness and power. Will use a SHO to wipe the floor. I know... As a multi decade Ford fan, I looked carefully at the SHO and even the same version equipped MKS. The Genny as a car, let alone the engine is light years ahead.
GM has proved in their trucks that GDI on a rework of old pushrods get the same mileage and power as the TTV6. Same for the Hemi on the Ram. And the Rams are not GDI. The Ram non GDI V6 without turbos is within 60hp of the Ford and gets up to real world mileage of 29+. Sure, not the same torque.
The new Corvette with the old pushrods design updated with GDI pushes huge horsepower and still runs in the 34mpg on highway.
Back to Ford which has better designs than GM or Ram... The 5.0 and 6.2 with refresh using tech like the TAU V8 in the Genesis, would blow away performance and reliability of the TTV6, lower cost, and as good or better mileage.
I am not against turbos since I own also for my business use Ford F350 King Ranch 6.7, and late gen 6.0 in F250. They are amazing. Diesels do so well will turbos. And this also proves a point. The 6.4 and 6.7 wipe the floor with the 6.0 in power, and fuel economy. Bigger is better... The rice burner "tuners" had invaded Navistar/Ford and created the 6.0. Too bad lessons have not been learned at Ford with the gas guys.
Track record shows the 3.5TT as a amazing achievement. That does not mean it is what should be used moving forward.