Just to add a simple stat comparison of Legendary Reaper versus Power Toast (Purple). Assume Blood Bath, Pain is Power, Elemental Elation, and Fire Fiend are all taken.
With Legendary Reaper (+99% Kill Skill Duration):
10/5 Blood Bath+5/5 Pain is Power (while on fire)= +575% Weapon Damage
5/5 Elemental Elation (20 stacks)= +60% Fire Rate, +100% Magazine Size
5/5 Fire Fiend= +50% Weapon Accuracy, +35% Reload Speed
With Power Toast (+80% Burn Chance, +53% Magazine Size):
5/5 Blood Bath+10/5 Pain is Power (while on fire)= +400% Weapon Damage
9/5 Elemental Elation (20 stacks)= +100% Fire Rate, 180% Magazine Size
9/5 Fire Fiend= +90% Weapon Accuracy, +63% Reload Speed
Legendary Reaper’s +175% difference in Weapon Damage is huge, and you get that for 14 seconds as opposed to 7 seconds. But that’s just one factor in each class mod’s DPS potential. You also need to factor in differences in Fire Rate, Magazine Size, Weapon Accuracy, and Reload Speed, which all affect your sustained damage over time. When you add in these other factors Legendary Reaper has -40% Fire Rate, -133% Magazine Size, -40% Weapon Accuracy, and -28% Reload Speed by comparison to Power Toast. Both class mods clearly bring a lot to the table so I think you have to try each out with the gear you have to really see the difference in performance.