I have always found the greatest stress on my windshield is when caught in turbulent conditions, such as passing a tractor- trailer and similar. It is the repeated 'slaps' of air hitting the windshield that breaks things, not a steady pressure.
Normally I reach up with my left hand and brace the windshield when moving around a truck as I know the buffeting is coming. Probably not the safest thing to do but then again getting hit with a loose windshield would not win any safety awards either so I choose to brace....
My windshield is ~10" taller than a stock Gen. 1 C-14 windshield and has not broken..... yet. :-)
Brian
I am going to guess that the original/factory design is probably fine for the life of the bike *IF* one only ever uses the factory windscreen and no topper. But a lot of us are using something larger or modified in some way (me included- I have an X-creen topper, which I love, but it is a significant amount of mass at the very top of the windscreen). Wind pressure is one major factor and the other is an extra mass which can greatly amplify bumps and jarring into the support structure.