The range dip may instead be because of lower battery temperatures (from all of the wind). Better battery thermal management at high speeds might help.
Higher energy density batteries will also comfortably overcome this problem. In the future I can see EV's that advertise fixed ranges, so that you don't get arbitrary range dips. They'll do this by over dimensioning the batteries and using better battery management software to make sure that more than 95-99% of your drives will be able to go at least the advertised range
Our Leaf could probably go almost 50% farther on a charge if we never exceeded 30 mph. 30-40 mph has a small penalty; freeway speeds exact a much larger penalty.
But I find that when I'm driving at lower speeds I'm constantly stopping and starting for traffic.
Around where I live there are many roads posted at 80km/h (50mph) with traffic lights every km or so. So it's accelerate to 80 for a couple minutes, then come to a complete stop, then accelerate back to 80. That's clearly what swallows up most of the range.