Every little hole that holds paint will have to be cleaned before the marker will shoot accurately again. This is very important in the field. A clean barrel normally has very few ports or none. They will self-clean much better than a heavily ported barrel. The stock barrels for a lot of guns are non-ported clean barrels.
Clean barrels are normally much louder than a quiet barrel.
This is a tricky question because there are some things to be considered, or in a paintballs case not to be considered. A paintball has ballistics that are drastically different from that of a gun. This is because there is nothing to keep the ball from tumbling. If a bullet were to tumble it could not hold it's accuracy. This is why it is spun as it leaves the barrel.