Power plays count for a lot, especially in the playoffs. Power play percentage isn't nearly as important as playing 5 on 5 in the regular season, but it shoots up in the postseason, while 5 on 5 starts to even out.
At the same time, power play percentage, and the total number of power plays, is actually more influential on winning games and winning playoff games than goals themselves are. Goals scored is actually a relatively weak predictor, while special teams seems to count extra.
But here's the elite stat for special teams: 20% power play. 1 in 5. Only four teams reached the mark in 2007-8: Detroit, Pittsburgh, Montreal, and Philadelphia. Three of the league's Final Four - Western Conference runner-up Dallas put up an 18.1%.
However, 20% really seems to be the magic number, because things tail off below that. Tampa Bay records a 19.3% and Florida a 19.2%.