No direct experience to your question and can't speak of 40d, but in US we have 35d which is equivalent to EU 30d. All engines are variation of N57.
On my stock 35d, the best I got was over 41mpg (US) or 17.5 km/l. My average during regular commute is about 33mpg (14km/l) and with light traffic about 36mpg (15.3km/l).
Which leads me to ask: Is your car running OK? Any shadow codes?
Putting a performance tune on this car will be a catch 22 with fuel economy. Under light (partial) throttle application, you will actually use less fuel, while under heavy use, you will use more - duh. First few weeks with new tune you will actually get worse economy as you will be pushing the car a bit more then usual to enjoy the new power.
The reason why partial throttle application generally yields better fuel economy is because you have to hold the pedal for shorter duration to achieve your goal (accelerate, pass, etc.) thus more time cruising at lower rpm.
However, there might be tuners out there who can build "fuel efficient" tunes which are specifically made to maximize fuel economy by potentially reducing power, making throttle lazier etc. Think of it as eco pro ++ mode.
https://www.racechip.us/fuel-economy.html