First, the CalSci
I had a +5.5" Cal Sci for something like 3000miles. Cal Sci advertises that their shield are meant to be sized and used in the lower two positions on the bike and I can say that is true. If you raise it to the upper positions the vent hole becomes useless and the 'wings' no longer shield the hands. I ended up covering the vent hole for most of the time I had it. I rode most of the time with it in the middle position. It also does not have a compound curve to it so it does not deflect the air the same as the stock or CB types. It also narrows considerably toward the top. Last, the 'wings' on the side won't work with the Honda hand deflectors.
It was an OK screen and for me was way better than stock. I like the screen to be just above chin level most of the time. The claim of smoother airflow just did not happen for me.
When my CalSci was damaged I bought a CB +6. I like it a lot better. I still ride with the screen in the middle position most of the time. The part I like about the CB is the compound curve, which puts the screen closer to me. The 'flip' works and does not make more turbulence for me than the Cal Sci. But, that may not be true for a passenger (I never ride with a passenger so can't tell whether it is good or not).
Size does matter on the NT, mainly because of the change in angle of the screen as it is raised. That is why I like the taller screens is because at the heights I like, they are still angled quite a bit, which makes better sense to me than to have a shorter screen and ride with it almost vertical all the time.
The failure point on these screens will be at the point shown on Phil's. The mounting bolt location will NOT be a stress factor for two reasons. First, the force on the shield is acting as a lever at the upper end of the bracket (where Phil's cracked). Second, the plate that the screws go through is the part that is taking all the stress. It spreads the stress out on the front part of the shield. The bolts could be very loose and a steady force would not cause any problems at the bolt holes.
As for turbulence and flexing of the shield. The +6 does flex a lot. As does the stock screen, at least for me. When I am in a 30-40mph wind, at 80mph, with semis on either side, the turbulence is pretty bad. When I first got the bike I thought the stock screen was going to get ripped off it flexed so much. The +5.5 CalSci and +6 CB flex a bit more in those circumstances. In 'normal' riding they don't flex much at all. Even at 90mph there isn't much bend to them. It is only in the high speed buffeting around trucks that things get weird.
If my +6 breaks, then I will get another one, same size. But, will modify the mounting plate to spread the load a bit more.