I don't think anyone has addressed your question yet... I would bet that it's been discussed somewhere, but I don't know where and when in the forum. You can try searching on "fender" or "mpg" in a forum title.
You would think the rear spoiler would have very little affect. The chin spoiler, unknown. The retro fenders would reduce the mpg some but I've no idea how much.
In the grand scheme of things, there's not a big difference between 84 mpg and 74 mpg.
Or the avg city/hwy if I remember correctly, between 60 mpg or 53 mpg.