A wrecked car, in general, is going to be worth 25% or so less... regardless if it is a quality repair or not. Granted, if the repair is just lousy, then you can expect way less.
Now, the Hellcat, as most halo cars go, is a bit different. The typical buyer for a Hellcat is someone who's a bit above average in terms of economic mobility. They typically don't buy wrecked Hellcats, regardless what the cost is. Most dealerships (even Carmax) typically won't keep these cars on the lot. Most dealers will take it on trade and auction it. You can expect anywhere from 30% or more of a reduction on the value of the car as a result. You can also expect a lengthy inspection process on the buying dealer's behalf because they'll want all the records, receipts, and verification that the car is properly repaired.
I honestly wouldn't even try and sell it private party.
As for buying a used Hellcat... I don't care if it is a private party sale, dealership or a budget dealership like Carmax: Insist on having the maintenance records available, and get inspected by an independent 3rd party mechanic that's familiar with Hellcats. Do not trust or rely on the dealership to make claims like, "If anything goes wrong, we'll give you a 30 day warranty..." or some other promise. Some issues, like former modifications and/or abusive driving, can greatly reduce the lifespan of many of the mechanical components and some of them are extremely expensive.
Trust me, you don't want to buy someone's used track toy that's pulled 200 1/4mi runs on 8,200 miles and expect that you're going to be driving it around and not be saddled with several thousand dollars of repairs soon. That's because I too am a hotrodder... and I only sell my nice cars when they need work that I don't want to pay for. I felt bad for the dealership that bought it, but my 1,000hp Mustang I traded in... it was about 2,000mi short of needing an entire lower engine rebuild after having been driven on 18psi for 20,000 miles.