I just made a HAI for
my car and I'm seeing positive results. It's hard to tell what impact the aero mods have because I installed everything at the same time, but I did a little A A B (no time for another A) test yesterday and today on the highway, and it seems like the unscientific results were a 3mpg avg. increase. I saw IA temperatures up to about 140F, which worried me a little until I saw that my engine temp gauge in the cluster looked the same as it always does. More quantitative results to follow.
My understanding before I tried this was that trying can't really hurt, its more about how your ECU will handle hot air. Depending on how it does your FE will remain the same or increase. Don't quote me on this, but please correct me if you know more than I do.