I don't know why you're doubting those numbers considering Pippen rounded up more or less the same numbers accross the boards in 94-95, the same era where scoring was pretty rough for perimeter players.
My problem with your argument is you're obviously exaggerating Pierce calling him "extemely efficient" when in reality his numbers almost mirrors Kobe which happens to be one of the least efficient perimeter player of his era.
Also, I wouldn't credit Pierce in Kobe and Lebron's subpar numbers in those series. Boston was just that good defensively as a unit. tbh KG deserves more of that credit. While Pierce was in fact a good defender and can hold down the forth on short occassions, he was never in Scottie's level defensively.
Offensively, yes, he had his bright spots but again, in the Finals, Pierce came out flatter than Kobe Bryant ( Don't believe me? Check out both stats on both NBA Finals encounter) Additionally, Ray Allen came though more for the Celtics in the clutch than Pierce evident by Ray Allen's excellent games 4 & 6 in the 2008 NBA Finals, coincidentally, Pierce had a poor game on both games.