I think what everyone is arguing over is some revisionist history. People plainly said that Mullen did not have the scheme to win more with less, and they documented it with his record in such games. So that means he needs better players. Well, he doesn't have a good track record of recruiting. So you arrive at the 9-10 game Richt comparisons. No one said he was a bad coach. He gets a lot out of his guys, but he has a limited scheme and less talent in a lot of situations. So our best bet for him to push his ceiling up is through recruiting.
I will say that it appears that he is able to get better QB talent than what he had at MSU. If these QB's pan out, it gives him some (not a lot) of leeway in other areas. However, he needs top 7 classes with QB's that pan out, if he expects to compete. That also means occasionally hitting the top 3 and preferably hitting the top 5 most years. He's also going to need to stay strong on defense.
So I'm not going to say he's capped, other than to say that his recruiting will cap him. So look at his record there and make what you will of it.