Dynamos are constant current sources. The trick to higher power is more volt and tuning capacities. Jens based his current design on this: http://www.forumslader.de/Schaltplan.236.0.html and pulls up to 10 watt out of a "3 watt" dynamo. What about your way? 2, 4 or more tuning capacities settings? Input voltage seems to be limited to 21V in your case?

The efficiency input-output you measure does not matter that much to the user. Common riders do not feel a 2-3 watt difference. Even tire presure can have more influence then that. But they will complain if the phone/gps/camera... is not fully charged at the end of the day. That's why Jens does not publish efficiency but wh per km or watt vs speed. I can quantify my needs in wh/day, but I cannot tell you if the dynamo is pushing a significant load or is idling. So... do you have a full plot of wh per km or watt vs speed for a commonly used dynamo?

About warranty: the EU legislation about this is published here: http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=URISERV:l32022 . It basically requires you to add a small paragraph to your warranty stating that you do acknowledge this state garantied warranty. (Like scott does it here https://www.scott-sports.com/global/en/support/warranty/ in the second last paragraph or Apple in the 2 colum of the first table here http://www.apple.com/ie/legal/statutory-warranty/ ) It is a very good idea to tell dealers how you will honor this required stuff because in the worst case they have to pay back the full price to the customer. Promoting a different warranty, no matter how good, will not solve this problem. This is a problem between you and the dealer, not between you and the customer and it is a question I was asked about. The fines related to this are considerable in Germany.