Like a mocking bird, 13-15k.
1. Most people won't appreciate the "lightweight" GTS interior instead of the nice leather interiors GTs came with.
2. Mods do not and will never add value to Mustangs unless it is ALL the way. Such as a completely built up FI motor. But, then again, you will never see the money spent in the selling price. I am sure you can find me a link where one sold, an ebay ad, etc, but from what I have seen they do not.
3. Most people would rather buy a bone stock GT and easily do those mods you have listed knowing the car was, in their minds, never beat on. That is one of the first things that come to mind on any type of modded, heavy or not, sports car like a Mustang.
You can charge 17-18k if you want but you had better be willing to sit on the car for awhile. Our gas guzzlers are only depreciating.