I think a lot of David Ortiz's numbers do bear out that he's been a clutch hitter over his career. Starting with his career postseason OPS of .962, which is higher than his regular season OPS of .926. That's not a big difference, obviously. But when you take into account the higher leverage of the games, and the higher overall caliber of the opposing pitching, I think I could come up with a formula that makes the difference a lot bigger.
Then you have his absurd World Series line of 455/576/795 = 1.372 OPS.