OP, just go to a dealer and get your car assessed. Of course they may lowball you, but it's better than speculating on here. Once you have a better idea, you can decide what to do.
If the cost outweigh the benefit, don't do it. Keep what you have. The v6 Stang is still a nice car to have...unless you have the extra money laying around, might not be worth it.
I got my GT 3 years ago at 24 years old. At the time it was awesome. Now, I barely even push the car hard these days, just use it for transportation. Just think will you ALWAYS need that power.
2012 Mustang GT Premium
BLACK on BLACK 401A. A6, HIDs, Security, Rear Camera+Reverse Sensing, Remote Start, Comfort.
(Planning on CAI/Tune then I'll be good)