It's not 5%. It's 10%. And the difference between -40% ATK/DEF and -50% ATK/DEF is huge: ~20% survivability AND damage.
I get where Aidoru is coming from. While this particular user had no need for advice about D'art or Sniper Shot, when a user comes and asks a very basic question that has a consensus answer, it's not helpful for him to get a bunch of seemingly contradictory responses from regular posters. You might know exactly what you're talking about, but new players won't, and they won't necessarily have the knowledge base to sort out the best answers.