I had the CB +6 on mine. IIRC it was on there for 16k of the 28k that I had on the bike when it was sold. It did not look like there was any stress cracks started.
Remember that this shield was larger than most on here, and was probably ridden at higher avg speeds
Interstates were usually at 85mph. Some of my back road trips hit over 100 for 15 to 30 min at a time. I-40 was horrible for truck traffic. When passing a truck I would notice the shield flexing at least 1" (the stock shield also flexed that much in those conditions). It also took some direct hits with some large marble size rocks on one trip.
Most of the time I rode with it in the middle position, sometimes in the 2nd from top or top and looking through the flip there was no distortion. Keep in mind the way these were made. CB used the same mold for all and just cut them to the height desired. So for the +6 almost all the extra height of the shield was in the 'flip'.
I also was not "kind" to it. At freeway plus speeds I would "slap" the shield to a lower position (when running over 90 there was much less drag with the shield all the way down and me 'tucked'
).
Would I buy another? Yes. If you are bothered by the cracks then do what I had planned. Make spring steel extensions to the mount brackets to better support the shield.
FWIW, this is one of the worst designed parts of the NT, the windshield brackets. The change in tilt of the screen when raising it is stupid, IMHO.
I would have liked to have tried one of the National screens, but, never had a need to replace mine before the bike was sold.
The Guzzi has a nice big screen on it and I added the Vario-X extension to increase the 'bubble'. Now I ride without any of the windstream hitting my hands, shoulders or the helmet. If all you want to do is move the airstream a couple of inches the Vario will work fine.