Of course the issue is perception. You can design any LED panel you like, but if your boss or the customer looks at it and they don't 'perceive' it to be bright enough then it's a failure. No amount of equations or theory can overcome this. If the panel is designed to be read by the human eye, then that is what it must be judged by. In the final act, perception is everything.

Having said this, if you need a LED to be at absolute maximum perceived brightness, then I agree that multiplexing should not be involved.

Multiplexing can be useful, however, when absolute maximum brightness is not required. For example if you multiplex at 50% duty cycle, then power is halved, but perceived brightness is (in my experience) at least 85% of the non-multiplexed case. Thus the multiplexed case has a higher efficency.