Your compensation would be based on the fair market value for a comparable car. So you could show them recent sale prices for a similarly equipped car that had FUSC.
There are no comparables. The supercharging is not transferable to a new owner so only older cars would be available with that.
Do cars with FUSC sell for $33-56k more than cars without it? No, so the insurance company isn't going to lay a golden egg for you.
How do you know that? I can show a direct cost analysis, so the comparable car issue goes away.
I recently went through something similar with an ICE car that got totaled. It was a collector's edition where only 500 were made. And they based the value on buying a non-collector's edition and then adding ~40% of the cost of the collector's edition package. (Which obviously isn't actually a comparable car.) I had to fight and find an actually collector's edition for sale, at a dealer, to get them to budge on the value. (And they wouldn't accept private party ads as they said I could be working with the seller to inflate the price.)
Funny. My car was totaled once. The other guy's insurance company was arguing that I could buy the car from ads in the paper at a lower price than what I was asking. lol
I ended up getting ~75% what I wanted, and probably could have hired a lawyer to get more, but then I would have netted less.
Clearly your issue was that you were willing to take less to end the hassle. But your issue is not really analogous to what I am describing.