Here is a reply directly from Pfadt that I received.
Pfadt provides adjustment in the rear bar to suit different driving styles, track conditions and tire combinations. On an otherwise stock car, we recommend to start with the middle position, and adjust it in either direction if necessary, based on the aforementioned variables. The idea behind this is to provide a "neutral" or "balanced" feel for a variety of customer set-ups. Because of this, a customer running a "square" tire set-up, like 295's at all four corners, can potentially achieve a balanced car using the softest of the three adjustment holes, because this wheel/tire combination would traditionally have more front grip relative to the way the Camaro was stock. A customer with the stock wheel/tire configuration may require the use of the middle or stiffest adjustment setting, to get the most balanced feel to his Camaro.