Today I got the car back. As already written, the oxygen sensor (lambda/O2-sensor) has been replaced. With a lot of wheel spin, the car now did 0-100 at 8,3 seconds, from previously 10,2-10,5.
I started the measurement in first gear, maybe I would get better results starting off in second. Not sure.
The shop said that they couldn't find any other faults, and emissions are good.
However, idle is still not the best. It's better, but not the best. The engine seems to miss here and there at idle.
Should I fork out for a new MAF, or could it be something else? The engine does run better, mind you.