I'm sure it did something for gas mileage. I'm also sure it was too small to notice. I think earlier in this thread, I estimated the gap sealed equaled sealing up about a 5" hole in the grill. That really isn't that big at all when it comes to surface area. People see a 1-2 mpg bump from blocking off the entire grill if they drive at high speeds most of the time. So, blocking off a fraction of the surface area leads to a fraction of the gas mileage increase.

In my case, sealing the hood gap also seals off a way for the air to get around the radiator. This improves cooling performance and allows me to run with even more of my grill blocked off. Is it noticeable from tank to tank? Probably not. But, I know it is working, and its a super easy mod to do.