MY PTBB win rate over 30k hands at 25c/50c is 10. I'm pretty sure that I'm not so good that that is the absolute maximum attainable (espeiaclly as this was before they doubled the stack sizes which must help the better players win more)
However why you would stay playing the $50 tables if you were consistently making 15PTBB/100 is more the point - of course you wouldn't -you'd move up and your win rate would drop accordingly.
AS for moving up - I'm at $4000 now and have just moved up to the $100 tables (at UB). Finding it hard work to be honest - I will have to tighten up considerably moving forward..