At what point is a Lifedrinker Warlock’s expected damage reduced by Sharpshooter/GWM?

At Level 12, a Pact of the Blade Warlock can take the Lifedrinker Eldritch Invocation, which states the following:

When you hit a creature with your pact weapon, the creature takes extra necrotic damage equal to your Charisma modifier (minimum 1).

Assuming a Hexblade Warlock with a 20 in Charisma, this translates to 10 bonus damage per attack. This enormous bonus means that every missed attack results in a significant reduction in damage.

The Sharpshooter and Great Weapon Master feats grant options of the following pattern:

Before you make [an attack] with [a weapon] with which you are proficient, you can choose to take a -5 penalty to the attack roll. If you do so and the attack hits, it deals +10 damage.

At what point is the Warlock’s expected damage per attack reduced by using the optional attack modifier of either the Sharpshooter or Great Weapon Master feat?

Assume a Hexblade, Pact of the Blade Warlock with the following characteristics:

  • At least 12th level
  • Lifedrinker Eldritch Invocation
  • 20 in Charisma
  • Using a pact weapon, either:
    • A longbow
    • A greatsword
  • … with the respective weapon feat:
    • Sharpshooter
    • Great Weapon Master

With the following variables:

  • Enemy AC
  • Attack roll and damage roll bonuses (due to spells, proficiency bonus, magic weapons, etc.)