I tried it out in VGC for a bit, and it would be nice if Flying types weren't half the meta (and Fire types another quarter). Honestly Grass types not named Ferrothorn (and Ludicolo to an extent) are awful in VGC right now, and Ferro 204+ Atk Ferro gets the KOes it needs anyway. I guess the damage boost means you can get the KO on Ogre with no Attack investment, but it's too hard to keep up with all these Primal Groudon running around lighting things on fire. The best calc you are going to get is a signifigantly reduced chance of getting 2HKOed by a neutral nature boosted Xerneas while holding Lefties: +2 252 SpA Fairy Aura Xerneas Moonblast vs. 252 HP / 52 SpD Ferrothorn: 92-108 (50.8 - 59.6%) -- 34.8% chance to 2HKO after Leftovers recovery and Grassy Terrain recovery.Would probably be best in doubles, so you don't waste the next turn switching out after setting it up. Tanky Grass types (Tangrowth, Roserade, Mega Venusaur) will probably appreciate the slight healing it gives, and the 50% boost to Grass moves as well to give them more offensive presence. Part of me thinks that offensive Grass pokes like Contrary Serperior could work pretty well with it too, but someone would need to test that out.
Fortunately the Flabebe line gets the move, so you don't need to have 2 Grass types on the field at once to use it.