That "article" was just a puff piece to get people talking.
I don't see how Saleen can do anything to a Tesla except modify the suspension and wheels. They can't "tune" the drivetrain because I don't believe they can get access to the core software that controls it. Why would Tesla allow that?
And surely any Model S that Saleen modified would not be supported under warranty by Tesla, so the owner would be liable for any repairs. Seems crazy to me.