I agree that for a round beam 115 lumens is rather limited, but as you get brighter it gets harder to set your light to see in front of you and not have too much light above the horizon. I think I can do quite well with my 13W HID that has a 12 degree beam (about 500 lumens), although even it starts to have trouble punching through the greater distances while I feel that at those greater distances it is apt to bother drivers. I want (but don't yet have the money allocated to purchase) a properly shaped beam light.

As for a 15MPH top speed, IME it doesn't take much of a hill to best that without much pedaling. I ride in somewhat hilly terrain, and I can often reach 25MPH by making myself more aerodynamic while coasting. Add pedaling to that and 30 often isn't too hard (when I don't reach the next curve too soon.) On a flat MUP, however, I could believe in a 15MPH top for moderate pedaling, although I still think that with a tailwind you may well exceed that a bit at times.

If I were riding on a MUP I would be very cautious about overdriving my lights. I was on such a path on Thursday for about eight miles (with perhaps heavy pedestrian traffic), and 22 miles of road riding, including with shoulders of less than ideal width and some hazards in them with the speed limit being 50MPH, and I found that the scariest part of the ride was passing all those pedestrians (most of whom were taking part in an event that included cycling on the path!) Those folks are not alert for cyclists passing them at several times their speed, and remember that you do not have anything close to the movement options that they have.

Sorry to ramble so, and I probably shouldn't have gone into such detail on paths here, but I do think it has some relevance to lighting. Hope this is useful to you and not too much.

Tor