The thing with exotics is you gotta look at the general theme of play it was designed for, not just what would make it better by your own standards. Sweet Business is basically designed to be an LMG and play like Heavy from TF2. It's lore tab and perks are designed to just let loose and hold the trigger down and just spray everywhere. Explosive rounds + firefly would be amazing, but wouldn't fit the theme. I don't believe Zen Moment would fit either. Rampage would fit it perfectly. The unflinching perk shouldn't be the masterwork and either come with it naturally or not at all. Armor Piercing rounds would fit well too. However, I believe the best possible solution to "fix" Sweet business would be a massive increase to non-precision damage and armor piercing rounds. It's Masterwork should give it rampage and mulligan. I believe that would increase the theme of it's playstyle perfectly.