I have a VStream. I run with it all the way up. I can look just over the top and have no helmet buffeting. If I get much higher, I start hearing some buffeting. Crouching forward a little, and looking through the absolute top of the windshield, I find is a very comfortable sight picture for me. All I hear is the front tire's road noise and the motor. No wind. That's nice. As some have mentioned, there is that envelope that the angle of the windshield will provide. But the results depend upon the riders position.
Remember...if you add risers and other devices to make you sit back a little and sit "higher", you are more apt to be in the wind and your wind shield is not doing what it was designed to do for you. I didn't like the stock, but the VStream is about 3.5 inches or so taller, and a little wider, and it also eliminated should buffeting. All this makes a big difference on long distance rides. I don't need ear plugs any more to protect my ears from the wind hurling around my helmet. Good luck with your choice.