I max out at 35mph with a 46 tooth chainring and 11 tooth cassette. I thought all I would need to go faster was a larger chainring. Turns out that the problem is the load on the motor from friction and wind drag.
Wind drag goes up dramatically above about 18 mph. The faster you go the more you pay in current draw to get each mph faster. With my bike on the stand with no load that same combination will hit 56mph at 58 volts. That tells me top speed is not limited by the gears.
With a 46 tooth chainring if I want to add any power at 35mph I need a cadence of over 110rpm. I not only don't like to pedal that fast, I can't add a lot with my legs changing direction up and down that fast.
A larger chainring can help with leg muscle efficiency but the BBSHD is more efficient at those higher rpms so you lose some efficiency when it has to use a higher gear. So the law of diminishing returns kicks in on multiple fronts.
Bottom line is above 20mph you are using more power for lower payoff and the cost is sucking down your battery fast. The faster you go the worse it gets. I typically ride at 20mph and that is fast for a bike. When traffic dictates or I am in a hurry and have enough battery I will push it up to 30+. That is where bicycles start to push the margins of safety they were designed for. If you want to go faster and stay safe think about starting with something that was designed for faster speeds like using motorcycle and moped components.