Won't the existing screws create a gap between the bumper and the lip, since the lip won't be able to sit completely flush?
No lie, I've been thinking about trying to improve the J's aerodynamics for gas mileage (ala
http://www.ecomodder.com), and adding something to direct the air a little further down would be a potential part of that. In all honesty, I probably won't do it, but I've considered it. WI winter wouldn't be friendly to flimsy plastic fastened onto a RWD car. I'd turn into a snow plow, hahaha.
While I'm at it, though, what is everyone's gas mileage like these days? I just filled up at 25.6 mpg, mixed city and highway. I've hit over 27 before (all highway), and I want to somehow make it to 30...hence the aero thoughts. Just for the challenge, really.