Thanks for the reply. That's still a theoretical reasoning. "Based on our current _models_, it must follow that c=c'." I can accept that. I guess part of a wider theoretical answer is that a photon is just an interaction in quantum fields, and that indicates there's nothing special about a photon that could limit its speed (as you imply.) What you're saying makes me think I should be looking for impediments for attaining speed, and it seems only (inertial) mass is that thing.

My question is if this part of the model has been validated experimentally somehow.

BTW, it seems odd calling a photon a zero-energy object.