Alan, if you ever get down to South Sound BMW they actually have a demo of the windshield extender in the store that you can try for a blast up the freeway and back.
That said, I tried it for a blast up the road and immediately hated it. It was so much louder I thought for a moment I had forgotten to put in my earplugs. The stock screen puts the main turbulence on my neck and chest. The screen extender put it right in the middle of the helmet which was way worse than stock.
As the screen is about 5 or 6 inches tall, try slouching down into the seat, lowering your head about that much while on the freeway and see what that does.
The urge to buy terrorizes.