Casey and Shelby3 are absolutely correct. The mods you've mentioned have a value to them, but most used buyers look at the age and condition of the car firat....that's what makes the car appealing and establishes a value. If the car is modded and in good shape, well taken care of, it will produce a higher price to the right buyer. However, there are still a high percentage of buyers that want bone stock or will accept basic bolt ons (tubes & filters, maybe cat back, maybe gears) but no true engine mods (like s/c). You have to find the right person to have a chance of getting the right price...that's always been the used car market philosophy.