Rhodes,
I had my ECU remapped (not my Mog though...) recently and I asked the same questions.
The answers I got were that a 'standard' ECU is programmed with a lot of safety margins and restrictions that actually can make the engine less efficient. Remapping includes minimizing margins and removing of some restrictions to make the engine run more effectively when used in a relatively normal fashion. I was also told that it's normal that improvement in mpg does not occur until you've used up at least 2-3 tanks of fuel. This because the ECU has to self-adjust to the new settings and your personal driving style.
As I said, this is not my own knowledge but the information I was given by the supplier.
-99 4/4, 1.8 Zetec, Indigo Blue, 2-seat, Suplex RS
-04 Citroen Pluriel, 1.6 Sensodrive, Met Orange
| 
