While fuel economy may have been a factor, in reality, they gained nothing. My 3.5 MKS had a 2.77 gear while the Continental has a 3.39. By turning slower, the 3.5 MKS got better highway cruising mileage than my new Conti. Overall mixed driving mileage seems to be about the same, though.
I am not an expert on the 2.7/3.0 engine but I have done a lot of research. It is made from Compacted Graphite Iron (CGI). Advantages over the aluminum 3.5 are added strength, greater stiffness, less noise, and the ability to have thinner walls. The 3.0 simply takes up less space than the 3.5. It was designed strictly as a twin turbo engine and uses Borg Warner turbos than generate 16-17 PSI of boost in stock form.
I loved the 3.5 ecoboost in my MKS but the 3.0 is a newer and improved design. It is a much quieter running engine and has even less turbo lag than the 3.5. The fact that it simply takes up less underhood space is also a factor since they use the 2.7 version in the Fusion and the 3.0 in the MKZ. The 3.5 ecoboost would not fit in those vehicles due to the dimensions and the way the turbos were incorporated into the exhaust system. The 2.7/3.0 family is a more compact.
I hope that over time the tuners can do as much or more with this engine than they have done with the 3.5. It seems that the potential is certainly there.