Christoph - it's only an issue if it doesn't keep the engine temperature in check, what temperatures are you seeing?
I know in Germany from experience that you generally have higher average summer temperatures than here in the UK (looking out the window on the damp view) if the air is warmer then it'll need assistance at lowish speeds.
I take it that you have bled the coolant system?
You could measure the the temperature that the radiator is switching at to make sure it isn't switching to early.
