Well, Cell's case it's not really that much different from Black's. I don't think it's like zenkai boosts stop once you reach a certain power level, but rather that the zenkai multiplier caps once you've accumulated a certain amount of battle experience w/ Saiyan cells. So Cell, who was already far stronger than Goku and Vegeta at base, probably could have ascended far higher if he had more time and an abundance of strong opponents.
Cell probably could have become the strongest non-god DBZ character, left to his own devices.
EDIT: Blegh, double-post. Well, I'll take this opportunity to ask this: How much higher do you guys think Goku's base power level was during the Boo saga than on Namek?