The mental aspect of winning a slam is huge, easily as big as the B05 factor.
Imagine players like Thiem, Anderson, Gasquet or Cilic playing a slam final against Djokovic or Nadal. We'll gift them a 6-4, 6-2, 4-1 lead. I still think all of them would choke away the match and eventually lose.
I am still in shock that Stan managed to hold it together to win his slams, considering the quality of his opponents.