Im sure it does make a difference. Ive heard of a fellow who put splitters on his front bumper to give it more downforce, and his tail would get light when he went around corners going really fast. This is the only reason id consider getting the rear wing that looks like this...
http://www.avogl.com/images/Boxster/Wing-020916-012.jpg
I like that one the best out fo anything ive seen because its not like a triangle wedge on the back but lets air underneath it and is thinner. Now if only I could find it online...