I cut the lip only off mine to improve buffeting and it worked. There is more wind noise but it is constant, I find this preferable to intermittant buffeting. The bike is also more stable in all conditions - especially in dirty air.
My method was different to Elvin's. I removed the screen and did the masking tape and marking the same as Elvin. I cut it with a jigsaw with a fine hacksaw blade fitted then using a fine file removed the cut marks and then the burrs. I finished off by dragging a sharp blade across edge and corners.
An Australian tested his standard bike in a wind tunnel with very interesting results, which can be viewed here:
http://gtr1000.yuku.com/topic/617/speed-wobbles-a-0-kph