Really? You don't think that the performance of the GT-R or GT350 could be bettered by someone building something based on a used car? Who said anything about starting with a new 5.0L Mustang? After all, Sage has a '12 model Raptor that he'd be starting with in the original example. I think that bettering the performance of a GT350 would be absurdly easy if you started with a 1-2 year old 5.0L Mustang. A blower setup, big brakes, and some high end shocks/suspension would get you there... and leave you plenty of budget for stuff like a cage to stiffen the chassis and give you a place to attach proper 5-point harnesses.
It's still used at that point. Not really much point in comparing used to new. Used is obviously cheaper.
Not sure why you would have to build something out of a used 5.0L mustang to beat a GT350 when you could just go buy a C6 Z06 for $35k.
Also keep in mind, in your example, the GT350 will hold value reasonably well.... and the brakes/supercharger/suspension/cage/etc... will lose 75% of their value as soon as you put them on. Hell putting the cage on is going to SHRINK your market and hurt resale.
You don't know the true price of a car until you sell it.
Note: nothing above should be used to make any claim that a Shelby raptor isn't a waste of money.