No, I do not believe that there are players that suddenly get substantially better in high pressure moments, but I do believe there are players who get worse. As other posters have mentioned, it comes down to sample size. Players do not have that many chances to attempt or defend against game winning Super Bowl drives. Are one or two or even four or fives drives really enough to judge whether a player can consistently succeed in that situation? Only in the storyline driven world that we sports fans create.