I ride with Schmidt dynamo hub for brevets, and the drag is not noticeable. There've been more than a few daylight commutes wher I forgot that I left my lights running from the previous evening. There's an occasional vibrational 'hum' that can be felt when coasting between 10 - 15 mph., but otherwise there's little difference in performance.

I would not take the 3 watt limitation at simple face value. The latest generation of dynamo LEDs harness reflectors inside the bulb to create some pretty impressive results. The debate nowadays has turned from whether the latest dynamo lights are bright enough to see or be seen, to whether the lights are too bright and pose a danger in blinding oncoming motorists.

I don't have an internally geared hub, but I do know that if you plan on using a Nexus 8 on a bike with drop handlebars, you normally have to kludge some method for mounting the rapidfire shifter that was paired with it. There are supposedly 3rd party bar end shifters for the Nexus 8, but I don't know how well these products perform in real life.