I think everyone's seen my aero by now, hard to miss...
Large vents in the front fenders seem to help a bit but I won't know for sure until I take it to a race track. The rear wing is almost necessary above 120, and can be felt as low as 50-60mph. I've also trimmed up the rear fenders to help them air out a bit as well. Currently I'm working on front aero which is not present and the bug understeers as a result. With the airdam and splitter in place the balance should be a lot better and I'll be able to dial in the rear wing properly.
For street use something above the rear window is best. A small strip like carbon joe did but a little bit taller, 2.5"-3" from drip rail to drip rail, would be ideal. Vents in the top of the front fenders might work for the street but I don't think they will be as effective, however they won't ruin the paint on your rear fenders.