I think it depends.
My son is a Honda nut. He will not buy (unless he can get it dirt cheap) a car that has been performance modded. He says if they spent the money to mod it, they "ragged on it" - his words.
Appearance mods could increase value on certain models but lower others. My son is dieing to lower my car and change the wheels - not gonna happen, at least not while I'm alive, I want mine left as close to orginal as possible.

 

i think modded cars appeal more to a younger person trying to buy a newer and faster car, at a lower price, because your car is really nice looking and worth quite a bit, i don't think it will have that much appeal as a modded car, unless you find someone that really likes monte carlos and wants a good starting place.

 

I did switch floormats in mine but only because I didn't want anymore wear on the originals, they are put up in a safe place. I "replaced" them with rubber #3s.