You are looking for the wrong thing. There is no rule that requires them to add one. Once your car qualifies for either the 2nd (WC-legal) or 3rd (IT-legal) quoted bullet, then you get to skip the entire rest of the GCR Prepared rules, including the one that makes you put in an SIR.
EDIT: incidentally, that means that any WC car or IT car gets to skip the rule that limits DP to 3000cc. So Tommy Archer could enter his WC Viper in DP! I wrote a letter about that one. But the point is that if IT cars don't have to add a restrictor, then neither do WC cars!
Or, perhaps you are saying that the WC rules (as opposed to our GCR) require an SIR (I'm clueless about WC rules). Well, if so, those SIR-equipped cars are running lap times that are WAY out of reach of an ITR car. Last year at Laguna Seca they ran 1:37.0 in qualifying. I can't believe we'll ever see an ITR car below 1:42. Unfortunately our region website is down so I can't find the track records for SCCA classes. [/b]