I think adding Devouring Plague to the “Screams of the Void” talent in Shadow’s spec tree would make the Mind Flay build actually feel good.
“Mind Flay: Insanity” is nothing compared to what it once was in MoP, and if blizzard doesn’t want to buff it, then adding DP to the ‘Screams of the Void’ talent would make it your DP>MF: Insanity window actually feel substantial!