There was a claim, many years ago, against ez, when speedy borders were put on the same bus as regulars. And you guessed correctly that the regulars were more speedy out of the buses than those who had paid. From the story in the DT they lost their claim. Did they pursue it? I don't know. Does it still happen? Perhaps not. Should you have a claim? Of course. The supplier has not fulfilled their contract.
Remember, on RYR, priority boarding does not get you a reserved seat. Those offer more leg-room and are at a premium. A reserved seat means you can linger and board relaxed; or does it include priority boarding?