It is not about terminal velocity racing down a hill it is about acceleration. If terminal velocity were important at all, it wouldn't be the magnitude of the terminal velocity, but rather how fast it is reached. We are not, in general, talking about bikes going as fast they they can down a straight ramp, but complex maneuvers with acceleting, braking, cornering, etc. Beside the handling issues, the most important characteristic is acceleration TOWARD terminal velocity, not riding at terminal velocity. That acceleration starts up again after every slowdown. So the net acceleration is what matters or weight acceleration offset by drag deceleration.