So I admit I'm a little confused. You ask about the powers of the Roman plebs, mentioning the plebeian assembly and the tribunes (not sure why the imperial tribunician power of the Principate is being brought up, but yes the tribunes are a product of the Conflict of the Orders). But then you seem to dismiss them. How are you quantifying "power" here exactly? Because the plebeian assembly and the tribunes are, in the opinion of people like Fergus Millar (not that anybody agrees with him), the two critical representations of plebeian political authority, and they were thought of by the Romans as emblematic of the role of the populus in the state. If those don't count what exactly are we qualifying as "powers?"