With the dailies rework, the daily lock on Reaper Tasks for Slayer XP will be removed while reaper points per week will be capped. This will allow reaper tasks to become a core Slayer training method. Because of this, I would like to see the Slayer Helmet (and maybe Genocidal perk?) work on Reaper tasks.
However, this bonus would only apply if you have the Reaper Crew achievement, to prevent cheesing DPS checks for your first kill on a boss. (Alternatively, just make Death not able to assign reaper tasks for bosses you haven't killed yet.)