Pay the money and get the Love the Drive model for your car. Windows up, wind deflector up, 75 mph..........interior is as calm as at 20 mph without either in place. You can literally have an easy, speaking voice conversation with your passenger at highway speeds, yet still enjoy the experience and openness of top-down driving.
That, and having a wind deflector will greatly extend the top down time on the calendar. I've driven in complete comfort (at city speeds though) down to 39 degrees with the heated seats and heater on and the top down.
Love the Drive makes an outstandingly well made product that attaches in seconds and can be removed just as easily.