Christoph,
I could be totally wrong here but my experience so far is, the fan only kicks in in traffic and at lower speeds. On free country roads i never heard the fan. So i think, it is more a question of airflow trough the engine bay than one of radiator capacity. Have you considered an additional airduct behind the front valance? I've seen them somewhere but don't remember the manufacturer. You could also remove the mesh behind the grill.
Spacers at the rear hood lock could also help.
Regards,
Beltempo
