There are various aspects of the CI/flight economy debate that I'm unsure about. If you were in an A320 and had that as a viable range of Machs, could you not have increased your flight level? Would that not have improved speed (or at least resulted in an Econ of .78) and improved fuel performance?
We are told to use the flight levels on our flight plan (and not ask to change them) - but often these are performance levels. If we are a tonne lighter, the next level up is often available early - and I still reckon that most times (other than with unusual winds), this gives a fuel saving.