Here are two good You Tube videos discussing the question, "Is it is worth modding a 5.7 Hemi or buying a Scat Pack."
Or buy a 5.7 that's been done with someone else's money (I did)! Lot's of opportunities in my neck of the woods.
Crunching numbers strictly regarding the car's 1/4 mile potential is rather limiting in scope. It's basically like saying that a newer 5.7 car isn't worth anymore than an older one since there's no difference in straight line performance. Yeah, it's great that you can make an older 5.7 car run with a newer 6.4 car, or even a 6.2 car but there's simply more to the equation than the 1/4 mile times, no?
Supercharged and beyond used R/T and SRT Challengers with 35-40k miles can be purchased in BC Canada for $23-25 US. Ridiculous jump in price to Scat's with similar mileage. What wasn't mentioned in the video was the tax savings which can be realized on a trade.