Personally, I'd say it's more than the windshield giving the Touring less high speed stability. Just about everything was changed from the other models - frame, tires, steering geometry and overall frontal profile. I've seen videos of standard windshield mounted Rockets running up to the speedo limits with smooth ease.
The Touring was just designed to meet a totally different criteria than the original Rockets and high speed running was not part of the equation while I believe that was a consideration in the original Rocket design, which is why it came out with a 140 MPH speed limit and V rated tires.