Returns used one with the original finish as well, I think - but they definitely painted a second car with a glossy finish. That certainly doesn't prove anything about the provenance of this car - I believe there were only a couple of full-sized hero cars for the first two films, and there are plenty of replicas out there.
As for the Six Flags cars, they were undersized one-seaters that had a driver sitting up in the hood. Bob Dullam might know a bit more about those cars - I believe he was a consultant when they were starting up the stunt show.