My feeling is impossible to quantify precisely, but if you are a 15+ mph rider, you're conceiving a bike that's too heavy. My cruising speed is slower than yours, around 14 mph, and I don't want drum brakes or IGH's because of the weight penalty. But I would not worry about internal friction. I doubt it makes much difference. I use a dynamo front hub on two of my bikes, and I can't discern the drag they create.