[Gelbster]

Bart,
Slow down.
The only piece of diagnostic data (not performance data) you've got is :
"The Airflow through the MAF was according to the Mechanic measured to be too low "
Please don't feed us info piecemeal and partial. If the Mechanic has done a complete test ,give it to us - the RKAT,FRA ,fault codes - all of it. Otherwise this is just random speculation which will waste time(ours) and money (yours).
Airflow - we need the actual value in gramms.
Meanwhile remove the airfilter and retest briefly .Big change ? Check/clean the TB .You are looking for obstructions.
Please tell us you don't have a K&N (or similar) filter fitted.
Without a Durametric connected and freeze frame data to post this will be difficult to diagnose logically and methodically.