I'll put it this way, there obviously must be some limiting factor, and that limiting factor is simply the amount of muscle that can even be built. Yes, high protein can be utilized by the body, but how much is actually being used for new tissue growth as opposed just being broken down and treated as sugar?
Besides that, who is to say that this, like everything else, isn't highly individual, as well? Maybe there are folks who can benefit from higher protein than others simply because their bodies partition nutrients a little more effectively. AAS increases protein synthesis, there must be guys out there who naturally have elevated levels, already.
All these factors make this a tough debate.