I think you're onto something rather than on something. Many great athletes struggle to perform in big moments. Dan Jansen comes immediately to mind, although, he eventually overcame himself. Greg Norman and Phil Mickelson, also. Phil figured it out eventually.
Barry Bonds was one of the worst playoff performers, of all-time, for his level of greatness. The first 5 post-seasons of Bonds career he batted in the 100's three times, and highest was .250. He got better later in his career but was still hit or miss, no pun intended. Perhaps, that will happen with Matthews? 6th time's the charm?
Our team has too many Barry Bonds on it. I don't think getting better physically is where the focus should be. Auston and the gang need to see a guy like John Smoltz's sports psychologist. It is between the ears. Mental affects the physical for all of us. Regardless of how talented a person is, if they aren't in the right head space, the physical will never shine through as it should. The Leafs are so progressive I have to believe they've done a lot of work in that area...I hope.