> the AGP Power Mac's don't share that limitation? Is that correct?

To my knowledge, no PowerMac shares the mentioned firmware limitation. With the mentioned hardware limitation, it depends. PowerMac G4 up to the Gigabit Ethernet (PowerMac3,3) model share it, while PowerMac G4 starting with the Digital Audio (PowerMac3,4) model and PowerMac G5 do not (they offer AGP 2.0, 1.5 V).