The 2009 KERS had disadvantages, which slowed the car not less, than it sped up.
For KERS they had to design bigger sidepods = bigger drag, worse airflow around the diffuser = worse aero, slower car
For KERS they had to design bigger cooling inlets and outlets = bigger drag, worse airflow… = worse aero, slower car
Because of KERS they could use much less ballast = less well balanced weight distribution with less optimal setting range, higher Center of Gravity = slower car, worse handling
KERS added much more disadvantages than benefits. It’s not just to push a button and you extract 3-4 tenths from the laptimes (though not on every track, on some tracks 1 lap of breaking wasn’t enough to fully charge it). The above described disadvantages slows the car with more than 3-4 tenths.
Look at the 2009 WCC result to decide which concept won; with or without KERS. Proof of the fail; the two best resourced teams, McLaren and Ferrari have finished only as 3rd and 4th in the Constructors. BMW and Renault had dropped KERS in-season. Williams, Force India and RBR although planned it, but never introduced, At Brawn it also was never introduced, they had preferred the better aero and weight distribution for a reason. KERS couldn’t even fit into the car without messing up those benefits which won WCC and WDC for them.
McLaren/Ferrari could have been good if KERS would have been mandatory. For 2009, the optionally allowed and strictly limited KERS was a big fail. Lot of efforts and money to lose big time.