Considering a 2% Double attack up as a 2% damage increase is unrealistic. Base stats will improve damage from every source (Autos, Ability, Burst) while double attack up will only increase damage coming from auto attacks. Even by equipping it to a Kami with no damage dealing abilities, you would need less than 25% of your damage coming from Burst for double attack to be better, and your burst does more than that. Not to mention, you get less HP too.