As of 3.0 we still don't get the full -20% but still -17% (which is essentially more than 1 stacking less). The way I think this is currently implemented is rounding the - 2% recovery time to some number related to the recovery speed (the game works with speeds under the hood) and then multiply it by the number of stacks. Hence the error from rounding is increased many times. It should be 2% multiplied by the number of stacks first and then transformed into recovery speed. This way when rounding you get a much smaller error.
Also I've just observed that the cone attack does not work properly in all situations. There are cases when the yellow zone displays targets as included in the area of attack but when the attack is performed they are not damaged.