Shame such an awesome weapon is now garbage tier. Seriously, compare it to every other damage perk that can roll on primary weapons and 15% is weak, no 2 ways about it. Every other perk offers (at maxed stacks where applicable) 20-25-30-33-40-50% damage. Every single one requires 1-5 kills, but the ones that require more than 3 usually have a way to skip some of the rest, like Kinetic Tremors or Firefly on a weapon with One For All. 15% is basically nothing, especially compared to alternative weapons like Multimach or Ros Arago if RNG blessed you with a good roll for that (still hoping for a Subsistence/ Onslaught roll to pair with Doomfang). The closest I can get to why they nerfed a pinnacle perk like this again would be because we couldn't apply as many buffs or debuffs back then or because of new exotics.