Cheap experiments:
Remove the mirrors, ride. If far better without the mirrors fit Aprillia* ones (do a search on here and watch the results page light up, £23 off eBay)
Remove the screen, ride. If better your screen setting is causing some of the turbulence (I say some because the mirrors are the main culprit). Experiment with screen settings (either with the stock mirrors off or with your bargain Aprillia Tuono ones fitted), I have mine on the lowest mount point with 2 x 4mm rubber tap washers under each of the bottom two mounts (again, search 'tap washers' here, pennies at Wilcos etc.), this slightly altered the screen angle enough to let more airflow in under the screen and eliminates most of the screen related buffeting.
*new BMW Adventure bikes have the exact sane mirrors fitted.