My best advice to you is that you are the only one that is going to be able to determine how much is enough. Or if you can find someone that has the exact same opinions as you regarding specific lights then you are probably okay to follow their advice. The ideal is to get your local LBS to let you take one for a test ride.
I would estimate that the majority of riders would find the Light and Motion Solo adequate for riding at higher speeds than you reference. Either you are conservative or have below normal night vision. I have found many riders that rave about the Light and Motion Vega and I purchased it for the convenience factor and group riding only. But at 85 lumens I personally found it barely adequate for group riding and totally unsuitable for solo riding. I have since purchased a Princeton Tec Switchback 3 and using it as a helmet light makes a good combination with the Vega on the bars. Recently I acquired a Fexin L2D CE Q5 Cree flashlight for my helmet and moved the PT SW3 to the bar and dropped the Vega. If I had purchased the Fexin before the PT SW3, I would have left the Vega on the bars and not purchased the PT SW3. At 180 lumens with effective spill as wide as a 2-land country road and only costing $63 and change delivered, the Fenix flashlight is a great, cost effective solution. You might want to try it as a helmet light to compliment the Solo. I do not have anyway of knowing if it will be enough, but it would increase your light by more than 50% and give you visibility into turns at a low cost. Search Fenix on this forum and you will find many discussions on the various models. Mine just started shipping at the end of October and is a white light. Some people prefer the more yellow Rebel LED.