My commuter has a Nexus NX-30 (i.e. the cheap one on Peter White's pages) with a single round Lumotec; my tourer and tandem both have a SON with a Lumotec Oval Plus and a secondary Lumotec. Resistance is minimal, although the Nexus is noticeable when rolling at high speed with the light on, which is not a problem for commutes in stop and go traffic, but could be an "incomfort" if I rode all night on rural roads.
As for lighting, I would compare the single Lumotec with a 8 to 10 W MR-11 light (i.e. a 10 W bulb on a brand new freshly charged battery is brighter, but after 15 minutes, the 3 W Lumotec is brighter. How come? Simply because the Lumotec and Bisi headlights focusses the light where you need it for highway riding, not in the sky. Whereas that "stray light" is useful for singletrack, it's not that useful on highways.
For your high speed descents, the single Lumotec won't allow you to ride at full speed unless the road is already lit. With dual Lumotecs or, better, with a Lumotec and a Schmidt E-6, you will be able to go faster, but you might not go at full speed.
Depending on your night vision, rain, snow, stray lights, etc., I would suggest a maximum riding speed of 30 km/h with a single headlight or 40-45 km/h with dual headlights, maybe 5 km less if it rains. In other words, I don't think the headlight would be a significant limitation on your ride.
On the plus side, the generator-based headlight doesn'T limit you in how long you ride, you never need to plan your charges, and the battery doesn'T die when it's -20 C. And if you really like high speed descents, you could supplement it with a 20 W single battery headlight with an autonomy of 30-40 minutes, which won't cost you an arm and a leg because it doesn't last that long.
One other factor that differenciates the Lumotec (or Schmidt) headlights vs cheaper ones is the current-limiting diode, which allows you to ride as fast as you want without killing your lightbulb.
P.S. I can't assess Peter White's taillights because I use Vistalite SuperNebula taillights. That way, I didn't have wires going all the way to the back of the bike, and my lighting system works whether I ride by myself, with a trailercycle, with a child trailer... or with both.