it's hard to compare what would have happened had Quinn gone somewhere other than Cleveland. He was a bit overrated because he was a Notre Dame QB, but there were things to like about him. It's not really Cleveland's fault that Quinn ended up being more interested in lifting weights than improving as a player, or that after he hurt his hand that he would be afraid to even look to throw a ball downfield... or that Eric Mangini hated him.
A lot of times a player's development has just as much to do with how the organization is ran. Not saying guys like Tim Couch or David Carr would have become franchise QBs, but a lot of experts point out how they spent their first few years running for their lives and it killed their confidence. I dunno about Couch, but compare how David Carr played his first few years with Houston (full of confidence and playing tough despite getting killed on a weekly basis) to his one year with Carolina (as soon as he even saw a defender, he would just drop to the grown so he wouldn't get hit). Hell, you can argue that Sparano and Henning played a part in Henne not developing since they didn't trust him as their QB. And on the flipside, a guy like Eli Manning was not viewed very favorably his first few years in New York, but Coughlin and Jerry Reese did a nice job building around him and building up his confidence. Same with the Packers with Rodgers, and the Falcons with Ryan.
Not saying that if we get Tannehill and he fails it's not because he sucked, but development is not a one sided affair.