MotorTrend says the wickerbill "makes all the difference" (see articles below).
On the street you're probably not going to see a lot of difference (other than the reduced visibility that you mentioned), but it is, in fact, a performance part that makes a real difference on the track. I don't have one (yet) but would like to get one eventually.
http://gmauthority.com/blog/2015/04/...or-trend-says/
http://media.gm.com/media/us/en/gm/n...amaro-z28.html