No stats to back me up but my impression has always been that Pierce, a fine field goal shooter to begin with, shoots a bit better in the clutch, even in the most tense situations, where missing could affect a whole season. OTOH, his FT shooting in the most crucial situations seems to fall a bit below his FT norm, which is pretty good but not up to the level of his FGshooting.
If I needed someone to make a clutch hoop I'd put him just a cut below the best Celtics clutch shooters from the past, Bird, Havlicek, White and Sam Jones. He's a couple of cuts below when it comes to free throws.