I have an ecotek CB-26P on a Saab 96V4. Last year I took the car from Brighton, UK to Zeist, NL which was almost exclusively motorway apart from the first two and the last mile.
Speeds on the way to NL averaged 50-55mph. The ecotek was fitted prior to the return journey (same route) were our speed averaged 55-60mph.
The fuel consumption on the way out I calculated as 31mpg which is the best fuel consumption figure I have ever got from the V4. On the way back I calculated the fuel consumption to be 33mpg.
I stress that the ecotek's purpose is to reduce emissions, any change in fuel consumption is a bonus. In any event, there are so many variables that will affect your actual fuel consumption, regardless of whether the device is fitted.
Please note, I am relating my own experience with the device. This was by no means a scientific test and I make no such case for or against the device. I own four Saabs, I have two ecoteks and only one is fitted.
Some people swear by them, some people swear at them and have an axe to grind. My view is undecided. I suggest that you look at the information available and determine your own view about the device.