Step 1) Switch the places of Empowered Imp and ISF in the tree. Plunging ISF deep into destro would preventing it from crapping all over Affliction and Demo like it does now.

(Step 1.5) Give Affliction and Demo their damage back in exchange.)

Step 2) Remove ISF's internal CD.

Step 3) Empowered Imp is changed to a 100% chance with a 12 sec cooldown. After casting the soul fire a debuff is applied preventing soul fire from being made instant cast for 6 sec (to prevent back to back instant soul fire retarded burst in pvp)

problem solved.