It seems that the dispell damage isn't fully, or at all, benefiting from spell damage or Shadow Mastery (Increases the damage dealt or life drained by your Shadow spells by 2%/4%/6%/8%/10%)

Here we have the tooltip for Unstable Affliction. The average total damage without spell damage or damage boosting talents is 1155. With same rules the average dispell damage is 1733. So one would take and average damage of 578 more for dispelling this spell.
Now lets see the actual damages:

The dmg UA deals as a DoT with 1265 spell damage to zero resilience: 469 per tick
Total damage: 469x6 = 2814

Here we have the dispell damage; 1765, merely 32 more damge with 1265 more spell damage and Shadow Mastery.
This bug is game breaking, especially at arenas. Taking about 1700 is covered with one heal, and the 5 sec silence won't feel very bad with that low damage taken for dispelling Unstable Affliction.