I've fussed with windshield height before. Mr. Jensee gave me a spare on condition that I cut it down because it was too scratched to look through. I cut it down to 9" above the cowling. I cut the other one down to 15". For reference, my line of vision works out to about 18".
I've lived in earplugs before (running chainsaws), but now I have problems with too much earwax. For the saws, I've gone to ear muffs (like headphones); and I tried some like that designed to install in helmets. So I cut up my POS helmet to put these in ... and they didn't work. Too much noise directly transferred to the muffs, like someone tapping on headphones with a pencil. My best solution has been silicone-style earplugs that clog the entrance to my ears rather than that stick in my ear canals. They're prone to noise leaks after bumpy hours in the saddle.
Since then I got a nice Nolan N70GT modular, which is quieter. (It's quieter still if I use sunglasses instead of the flip-down faceshield. Dunno why, but it sure is.) Now, with the 15" screen, I notice a lot less noise if I duck my head about 2 or 3". Since that windshield is getting cracks, I just bought a new Slipstreamer (21" above the cowling). I just did a short test ride, and now it's getting to the point that I really don't need earplugs for anything less than hours at a time.
The upshot is that the boundary between noisy and less noisy is with the top of the screen right across my line of vision. The 15" was about 3" below that, and the new screen is about 3" above it. I hate looking through windshields, but the taller screen is a lot quieter. I didn't get too much buffeting on this short ride, maybe due to using a suitcase on back instead of the trunk. The short screen is great for wicked hot days. Like I said, compromises.
I'll have more to say after a month-long ride coming up. I don't think I'll miss the wind since I'll probably freeze my butt off. I know I won't miss the noise.