I think an Ultragauge is going to benefit you quite a bit. Plus if you sell the car, you can just put the UG in the new car. (You can sell it to me by the way.
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I don't think removing the spoiler/wing is going to be of much benefit for you, there are some graphics on the site somewhere around here that show a spoiler is actually beneficial in helping with the boat tail "effect" without building an actual boat tail.
What I think is going to help you most in terms of physical mods is an air dam, belly pan and rear diffuser. The best thing about these three mods is they are mostly unseen or they add to the racing "feel" of a mustang. They are also completely reversible.
Welcome to the board!