The buffering effect is mostly caused by the vaccum effect behind the windscreen. I eliminated that effect by putting some 4.5-5.0 cm of space between the Dakar windscreen and the LC4 Adv windscreen.
It might not looks the best, but it works to eliminate the vaccum effect at high speed.
By adjusting the "tilt" of the F650GS Dakar windscreen with different length washers, you can easily find a windscreen position where you might be able to get rid of your buffering problem.. you might just need to drill bigger holes on your Adv windscreen to allow flexibility for mounting points.
Make sure you have plenty of supporting members for the mounting points of the GS Dakar windscreen, it will flex your Adv windcreeen at high speed.
Here's the original thread from over a year ago when I first put the GS Dakar windscreen on my LC4 Adv