I went from naked to a Buell flyscreen, it does make a difference.
It helps take some of the buffeting you feel and moves it up. Depending on how tall you are, it can move it to your helmet but it does take it off of your chest and mid/upper body for the most part. Anything above 70 will still move you around a bit but it helps. A lower bar position helps too as you're not in the "sit up and beg" position.
As far as theft goes, usually the bracketry is a bit complicated for it to be a "quick theft". Anymore I go by "if they want it bad enough, they're going to take it" mentality; something like that I don't think is a big deal. YRMV.
-Randy