i dont know about saturns but ur pretty much on the money. the SAFC/VAFC is a piggyback its not an ECU or a standalone. it intercpets the signal from the sensor makes the adjustment needed to be made and sends it to the ecu with the new signal.
if your ecu doesnt read boost even with the SAFC then u either need to find a company that can make the proper adjustments to ur ecu (like JWT for nissans) or get a stand alone.

good luck