He hasn't. He wasn't good in man as a rookie, but the team played a lot of zone so it was covered up. He's not good with his back to the QB. In 2010, the team still had Sanders and Meriweather, who knew what they were doing back at safety (even if Meriweather didn't always do what he was supposed to), so they could help McCourty and limit the damage of the mistakes. In 2011, they had a bunch of slappies at safety, so they couldn't cover for the corners, and the mistakes of players like McCourty and Arrington were on full display.
Teams got the book on him. We saw that over the course of the past two seasons. He's solid if he's facing the QB. He's too succeptible to the double move on the outside, and he doesn't quite have the high end speed to recover or to stay with the burners of the league, so he's half a step behind all too often at corner. If he had an Earl Thomas back at safety, he'd probably be fine again at corner, at least as a CB2, but he's got Chung and Gregory, so he's exposed.
McCourty's KR TD as well as his run down of the HOU KR to save a TD says he has plenty of speed. He also ran a 4.3 40 yard dash at the combine. Not disagreeing with the rest of your analysis though.