I wish you the best of luck with getting nearly $6,000 off the price of a new car. When you figure out how to do it, please, let me know! Not gonna happen.
Ford's "scale" that they are using is pretty close. Normally for every $1,000 financed it is $20-$25 tacked on to your monthly payment. I believe in the "scale," it covers the total monthly payment/amount financed. At least, when I helped my brother purchase a car, that's how it worked.
I've never leased a vehicle, but I do believe that the payments may be lower when leasing, however, the downside to leasing is that normally there's a mileage limit on the lease, and if you decide to purchase at the end of the term of the lease, you're usually paying more for the car than it's actually worth. You also need to be careful of the terms of the lease, because some of them require a "balloon payment" at the end of the lease if you choose to own.
IMO, leasing is alright if you don't do a lot of driving and/or want a new vehicle every 3 years, but in that case you always have a car payment. With leasing, you're also not getting anything for your money in the end. You're basically just p1ssing it down the drain. You wind up paying (example) $2,500 down up front, then wind up with garbage like "security deposits," and your $2,500 quickly turns into much more. You put forth all this cash, and in the end, you have nothing to show for it. By the time you get done with all the surcharges that go along with leasing to get the lower monthly payment, you could've put forth that money towards purchasing the car and probably wind up with the same monthly payment, all the while having some equity in your vehicle.
IMO, leasing is a waste of money, but to each his/her own. If all you care about is getting into a Mustang, go for it, but if you want to get into it, modify it, and drive it a lot, then leasing isn't going to be the best option for you. Personally, I would never lease a vehicle, but that's me.
Hope this helps a little.
In either case, you're right, at least you won't need to mod the stang, I'll be buying my supercharger around end of January or so, still haven't really decided on what I want though. Roushcharger if I want to keep factory warranty, or another supercharger and some headers if I wanna say screw it and go all out. 
