After reading this thread
I bought a like-new tall BMW shield from another K-Bikes reader. For the record I'm 6'2" and 240 lbs, my seat is in the tall position, and my bars are on the lowest position.
The tall shield is only about 2" higher than the short one, as you can see from the pictures. There's no additional flip or curve or extra width.
At first I was really aware of the extra height, and it felt like the top of the shield was a lot closer to my face. After an hour of riding I was totally used to it.
With the short shield I pretty much always rode with it up just a little, or else the airstream would hit on my neck and shoulders. With the tall shield that's gone, and I kept it all the way down on the backroads. (I like my helmet out in the airstream, I find it the most comfortable and quiet.)
On the highway I put it up a few inches to quiet the wind noise, and honestly I don't notice much difference between the shields at 70+, probably because I just don't put this taller one up quite as high.
After washing the bike I took a quick freeway spin to dry it off and put the shield all the way up to try it out. I normally only do that in the rain, in the high position I notice a distinct downdraft on the front of my helmet, which pushes the water off nicely. Again I didn't really notice much difference between the shields here, I can still see over the top of the tall one just fine.
So overall I'd say it's a modest improvement. But then I wasn't all that unhappy with the short shield. Both seem to me preferable to the Cee Baily shield I tried once, as neither of the factory shields induce the buffeting and turbulence I got from that one.