[jimlantz]

I think the instructions state most effective at lower volume settings.

True, the faster you go, the cabin gets noisier.
You roll down a window, the cabin gets noisier.
Open the sun roof, the cabin gets noisier.

The feature is only to keep the volume level above the ambient noise.

If you have the radio turned up to a high volume it will be above the ambient noise with needing volume compensation.