>>31229349Boosting item's don't effect the BP of moves, just the damage output. A Calm mind boosted special attack with no item is no different than a Specs boosted special attack.
+1 252 SpA Latios Draco Meteor vs. 0 HP / 0 SpD Abomasnow: 364-430 (113.3 - 133.9%) -- guaranteed OHKO
252 SpA Choice Specs Latios Draco Meteor vs. 0 HP / 0 SpD Abomasnow: 364-430 (113.3 - 133.9%) -- guaranteed OHKO
What affects BP is abilities and field conditions. Abilities like Adaptability,Steelworker,Iron Fist etc modifies the BP of moves alone. LO doesn't alter the move's BP, it just alters your attacking stats which makes the moves hit even harder. People only run life orb with Sheer Force pokemon because the moves that get boosted from Sheer Force don't give you Life Orb recoil.
Rain,Sun,Psychic,Grass, & Electric Terrain boosts the power of moves with a required type. Tapu Lele's Psychic would get the STAB bonus first for being a Psychic pokemon, than the Terrain boost next for being a Psychic move. Damage boosting items if anything are just icing on the cake. Because if your using a STAB/weather/ability/terrain boosted move(even when all those multipliers are used on one move), the cake is already set.