I do not have the E-Werk, but I often read, that it is more powerfull than the USB-Werk.
According to several tests the Dynamo Harvester Plus is also quite strong. Nevertheless, the Forumslader still seems to be the most powerfull loader on the market. There are several versions. The latest version is the strongest, but also most expansive. The older 12V+USB-Version can easily built up by yourself and is much cheaper and not that less powerfull.

There appeared a series of independent tests of several different loaders on the homepage of Forumslader.de and in the independet internet magazin Fahrradzukunft with a series of articles ("Steckdose unterwegs 1-4", translates something like "power plug on the road 1 - 4".) If you have any questions concerning the translation of the test results, just ask. To answer your question: 3 W@20km/h on a 622-weels-bike is about the level which can be expected with the USB-Werk. The Forumslader easily reaches the 3W-level, from lets say already 12km/h. I expect, that the Dynamo Harvester plus also provides 3W already at a speed essentially less than 20km/h.