While it is posable to do a big number given enough tries, I have done over 40 myself, you can't beat the proposition. You very well may have to handicap some of these players to higher numbers, it is very very hard to keep up any kind of good average. It is amazing how many racks you may score a zero due to a ball rolling up on the cueball or have to shoot a three ball combo to make your first ball.
That is the beauty of the proposition. A player has no concept how hard it is to even average 5. There is a big different having players just trying to set a high number with multiple attempts and betting on every attempt. The proposition will break most any player even just changing the fixed number by a ball or two.
The best example I can think of would be John Schmidt. He has run over 100 balls probably a 1000 times. Do you think it would be an even money bet for him to run 100 on demand for an even money bet? Im am not talking about one bet, it's a proposition. On an even money bet, the player has to do the proposition around 66% of the time to make any kind of dent in my pocket and even then he is just grinding out the money and can lose it all back plus his bankroll pretty fast. Every time he fails he has to do it twice to get back to where would have had he not failed. Even a great player can't keep that up. Quiting ahead, if you are lucky enough to get ahead, is usually the only way to beat most propositions.