As drag increases there is a cube relationship to the power required to overcome that drag. As the curve becomes more vertical the required HP to overcome drag becomes infinite. Its been years since I took a physics class but I think that's close?
You have the cubic relation correct.
V^3 = HP/k, where k=(drag coef * density of air)
However, from this equation we can see that as HP increases, then velocity will increase. Obviously there are practical limits to the amount of HP that can be applied. But the limits to velocity are not because of the aerodynamics of the vehicle.