Talkig about AC/DC Dynamo Converters, efficiency should be regarded, imo, as the ratio between the "active power" that is possible to "extract" from a dynamo at a certain speed and the "active power" made available after ac/dc conversion to its output.
Now the point:
since based on my empirical tests from a dynamo it's possible to get 6W at 20km/h and since the output of an e-werk at that speed is less than 3W is it correct to say that the efficiency of the e-werk is less than 50% ?

Below chart courtesy of Fahrradzukunft https://fahrradzukunft.de/


LEGEND
- Max Dauerhafte USB-Leistung = max permanent power at USB output [W]
- Fahrgeschwindigkeit = speed [km/h], all data based on SON28 assembled on a 28” wheel

I would not agree because it is not clear how much input power the converter needs for this 3W output from the dyna hub. But this would need a discussion of the behaviour of a dyna hub which I guess is done in other threads or forum.