Is there an uper limit to muscle growth? probably not anywhere near what any of us have been able to achieve in my opinion. Consider that none of us fully understand our bodies, we live in a constantly shifting environment, we have have other interests and obligations that we give attention to, etc. Then the only logical is that no one exists who has actually reached their own.
Not to mention that evoultion in supplement quality, training methods, foods, environment, and technology will push whats possible to achieve whether drug free or not upwards over time.
We are nowhere near what we will become