I think what needs to be understood about "ceilings" in terms of quarterbacks is this.....
...an evaluator or coaching staff must first establish how likely it is that a player reaches whatever his ceiling may be based on mental capacity, instincts, speed at which he processes information, and feel for the game.
An example would be a kid like Ryan Tannehill that may have a ceiling of 9.5 on a scale from 1-10. However, based on intangibles from the neck up, most likely only reaches 70% of his physical ceiling. He'll never be a 9 in other words. He's only going to reach 70% of that. He's going to be a 6.5 or a 7 most of the time because of inconsistency. It's who he is.
On the other hand, a kid like Andy Dalton may only have a ceiling of 8 on a scale from 1-10. Based on intangibles from the neck up and ability to process information quickly, he's likely to reach 90-100% of his physical ceiling. He's going to be a steady 7.5 or 8 because of his consistency.
That's why evaluating QB's is so complex. Once you establish what you think a player's ceiling is, you then have to determine how likely it is that he ever reaches it using the determining factors.
Kirk Cousins' ceiling isn't at high as Robert Griffin's, but Cousins is going to play to his ceiling (potential) more consistently. Even though it may be a lower celing.