Mastery of the Arcane should grant increased effectiveness of buffs. It is not affecting the effectiveness of potions, charges, or Necro pet buffs.
I self buffed the Necro pet and tested the damage received by the pet. I specced into Mastery of the Arcane 7, resummoned pet, rebuffed pet, and tested damage received by the pet and there was no change.
I also tested using stat buff potions with and without Mastery of the Arcane 7, stats given were the same in both situations (was well below the buff cap as well).
I self buffed the Necro pet and tested the damage received by the pet. I specced into Mastery of the Arcane 7, resummoned pet, rebuffed pet, and tested damage received by the pet and there was no change.
I also tested using stat buff potions with and without Mastery of the Arcane 7, stats given were the same in both situations (was well below the buff cap as well).