In my mind there is no way any tuner can achieve better mileage than the carmaker, especially when hp goes up.
Carmakers spend millions and millions on labs/tests/innovations/personnel (some of the best in the world) etc in reducing emissions and consumption while at the same time maintaining efficient and high output combustion engines. They have to meet some quotas or they are heavily fined.
No tuner with limited
resources can be any better than them on consumption, especially via a small tuning box, and that's that.
Its catchy to be advertised as such but it simply does not work that way and really it should not be advertised at all if they want to be credible.
More output means more consumption, unless you want to drive like a snail or with an egg on your gas pedal.