Certain metrics would suggest that other bullpens have outperformed the Kansas City Royals pen individually during that span, but none has been better during that duration.
During that span, the Royals lead all other teams in WAR, ERA, WHIP, and tied for the league lead in LOB% and saves. They have a significant lead in ERA- (72 compared to the next closest at 81) as well as being top-five in FIP, xFIP, and K%.
Despite this, Fangraphs projected the 2016 model Kansas City Royals as the 19th best bullpen, according to WAR, in baseball.
Much of this has to do with the loss/regression of Greg Holland, the loss of Ryan Madson to free agency, and a lack of confidence in Wade Davis‘ historic 2014-2015 awesomeness.