I'm with cycocommute on this one. I just upgraded my homebrew system from 12V to 14.4V and it's a big difference. While I was at it, I also tried out a number of different bulbs at the new 14.4 voltage. Some interesting findings:

• A 20W MR16 spot (est. 666 lumens @ 14.4V) completely blew away a 20W MR11 spot (lumens unknown). Granted, the MR16 was a highly efficient Philips EnergyAdvantage, but the MR11 was also a high quality Philips bulb not a cheapie. In other words, the best available in MR11. The MR16 appeared more than twice as bright to the eye.
• A 30W MR16 spot (1152 lumens) appeared somewhat brighter than the 20W MR16 spot (666 lumens). Noticeably brighter, but not night and day. That one kinda surprised me. Yes, the center of the beam was quite a lot brighter, but that tight little 8 degree beam was already so bright that increasing it by 50% didn't really seem to make as much difference as you'd think.
• A 30W MR16 24-deg narrow flood (1188 lumens) blew away the 30W MR16 spot (1152 lumens). Throwing all those lumens into such a tiny 8 degree beam ends up being kind of a waste. Spread it out and you've got a LOT of light on the road. Enough to illuminate the roadway even in driving rain, something I have to deal with often enough here that it's pretty important. By the way, this is only incrementally brighter than the 1080 lumens I was getting out of my previous 35 watt bulb at 12 volts ... but overvolting allowed me to get more light from a smaller bulb and step down from bulky 5000mAh battery pack to a much smaller 4200mAh pack.

Back to the OP's question: given a typical beam pattern I would find 390 lumens to be just barely enough to illuminate the roadway for a high speed downhill in total darkness. But only if it's dry out with minimal oncoming car headlights or other glare that impairs my night vision. If it's raining I would want a minimum of twice that, or else I'm going to have to be on the brakes and keep my speed under 25mph or even slower if there are a lot of intersections and conflict points. For reference, I should add that I'm over 40 and don't have the night vision that I did at 20, but probably a lot better than I will at 60.