Just a few comments...
1) If you've never taken a car to the dragstrip, you and your GT should do it. You may get hooked or may not, but it is a life experience not to be missed. And once you're there, I think you'll understand better why some people get so obsessed with the need for speed...meaning more power.
2) What's your root purpose/goal, in terms of the car? An investment, or a means of transportation, or a hobby, or an expression of who you are? I know that sounds kind of psycho-babble-y, but in my opinion you can resolve the money dilemma once you figure out exactly what you want to do with the vehicle.
3) Is your retirement in good order? I'm pretty laissez faire when it comes to what you do with your money, but if you haven't got a good financial plan for retirement you should put that $15K towards it and not the car.
4) The reality is, it's all dust and rust in the end. Rust never sleeps, as Neil Young so famously copped that line from an old commercial. Your life and mine, your Mustang and mine, are just great gifts to be enjoyed while we have them.
So let's do it! :wavey