It would be interesting to test D1 up 100 km/h and see how it performs in terms of high voltage at AC terminals (100V approx at no-load conditions) and overall efficiency.
Furthermore don't forget that a very common 3W6V dynohub can easily deliver up to 5 to 6W at 20km/h. So an output of 2.5W at 20 km/h is roughly the same as most of other similar devices on the market.

Cheers