Higher voltage is irrelevant, if the load is high enough and the converter has no limits then the dynamo will stay at it's nominal voltage (6V) regardless of speed. There is misconception that as you go faster the dynamo speed will be greater - only true if no load is upon the dynamo.

You are absolutely incorrect in your method of measure but completely correct in expectation. What matters is whether the phone is charged - correct, how well it charges depends upon how much power comes out of the dynamo and how much is lost in conversion (the efficiency).

Where this matters is that hard long day in wind, uphill, on a bad road. Your cache battery is dead and your speed is much lower than normal. The dynamo wattage is lower. The converter offering the higher efficiency rating has the best possible chance of device charging.

Cache batteries lower efficiency by 30% but enhance convenience. Better to use a USB powerbank with passthrough because you only get 500 recharge cycles, if you're lucky.

D1 protection begins at 21V.

Minimum loss from using an inline darlington shunt (Jen's design) is 0.7V/8.4V = 8.3%

This is the trade off between efficiency at lower speeds with a device attached and higher speeds with no device attached. I began with the latter and it wasn't enough for arduous touring conditions.