Theory about flips and lips
I'm considering cutting off the "flip" and the "lip" on the OEM shield. Hadn't thought of punching a hole in its middle (as above), but may do that too, as it should improve (laminar?) flow. Then, I'd supplement the OEM with a bigger, cold weather/long trip screen, maybe the Givi or MRA.
Here's my analysis (at 5'8" with seat in the lower position): The OE screen creates a high frequency vibe/bobble to my Arai XD4 (and also, but to a lesser degree, to my HJC full face "regular" helmet). Moving the screen up only changes the speed at which i experience the buffeting. My (too many decades) long experience with various OE and aftermarket screens leads me to conclude that screens with those concave "flips" to direct the air farther outward (bigger still air pocket) and/or "lips",, those beads at the screen's edge that must have been specd by company attorneys (maybe they're thought to reduce the risk of lacerations in case of a crash?---that both of these design features tend to create turbulence.
My crude tests of this for the S10 were to ride for a while with two different smooth shields on hand that were made for the BMW F650 GS, and that just happened to (sort of) fit. Both of these helped----some. The smaller one yielded the most improvement. One is approximately the size of the OEM, but smooth with neither lip nor flip, the other is no more than a flyscreen, and also nearly completely smooth along its outer edge. Running my bike yesterday both in town and on the interstate (up to 80 mph indicated, very light variable winds) with NO screen, I had NO turbulence, but I did get too much wind flow/resistance for relaxed long distance, high speed riding.
These "tests" tell me that the flow can be cleaned up completely by eliminating a shield, and that the flow can be improved to some degree by using a shield of a different (simpler) shape that is no larger than the OEM.
So at this point, I see no downside to modifying the OE screen, as it's not acceptable to me in its present form. I'll probably start by cutting away the inch or so to eliminate the lip and flip and then test the result before cutting more. My reluctance is that I don't want to botch the job.
Does anyone have ideas about how to cut, sand, and polish to get a decent result? If I attack it with a hack saw, will I be able to sand it smooth? Would it be better to grind away that inch or so against a grinding wheel?