It's pretty simple really
The power of thorn is the DoT and how it negates your recovery stat and makes it so you can't even start to recover until the DoT is done
Solution
[b]Make the Recovery stat lower the duration all DoTs last the higher your stat is[/b]
Not only would this nerf Thorn slightly without destroying what the gun is but this would add a hole new level of value to the Recovery stat, which I feel isn't as valued as Agility or Armor.