One of the car magazines tested the 2.7 versus the 3.5 and to their amazement they found the 2.7 a little faster 0-60 if I recall correctly.  I do remember they were quite impressed at the little engine.  I have a 3.5 ecoboost and like it so far but I have been reading that Ford has had a problem with carbon building up on the backside of the valves and if you use a fuel treatment it can remove the carbon but get really hard on the turbos when the carbon goes through.  Supposedly Ford is working at trying to develop a fuel treatment that they can recommend.  At this point, if they find out you used a non-approved fuel treatment they can void engine warranty.