Not sure why everyone uses axial fans for cooling the extruded filament.
A radial blower and a flex pipe seem much more appropriate for this task.

With the flex pipe the stream of cool air can be precisely directed at the tip of the hotend and a small ( 12V, 125mA ) radial blower creates enough pressure to push the air through the flex pipe.

First tests show very promising results. Now I can maintain 245°C on the hotend with full cooling ( without the head wrapped in kapton tape ). Also the extruded filament seems to be cooled much better than with the standard 40mm fan+nozzle which I used before.