I tested it myself a little while ago. Tried 4 tests, each about 30 non-critical hits from lv 14 Nightmare Hound. I took the average of each test by typing the numbers in excel and using science!
No pots
I took 176.62 dmg average
Potent Nightmare potion (should be 8% less dmg)
I took 190.97 dmg average
Strong Centaur Potion (should be 6% less damage to centaurs but no change here)
I took 176.11 dmg average
Weak Outlaw Potion (should be 0% less damage to outlaws and no change here)
I took 174.42 dmg average
So the nightmare potions did 8.12% more damage to me when they should have done 8% less damage to me.
The centaur slaying did 0.29% less damage to me (negligible RNG).
Weak outlaw potion did 1.25% less damage to me (odd but maybe just negligible RNG).
I have been using these potions since launch thinking I was a smart player. All this time with them taking 10+ slots in my bags. How sad.