Many of us have done better with the Ecoboost. A lot depends upon how you drive it. Over the 5 years I owned mine, the non-towing mileage was about 19 mpgUS, and most of that was with a pair of kayaks on top. But most of our driving was towing an ~8K trailer, and there the long-term mileage was only 10.2 mpgUS.
As you say, when in boost, the mileage is no better than any V8, but it is possible, with care, to stay out of boost when just cruising around. Admittedly, I do drive like the old man I am.
That, and the fact that the Nissan XD is a really heavy truck (compared to the F150HD) suggests to me that it will have worse mileage, more in line with the big V8s in the HD trucks, like Jeremiah said.