The Facade DD Roost +coverage attack on Mega Altaria seems better with max SpD and max HP as opposed to max HP and max Atk imo. It seems like it would be really good since SpD Megataria checks or even hard walls and sets up on many really threatening or just ubiquitous special attackers such as ZardY, Keld, Thundurus-I, Rotom-Wash(Facade says fuck burns), Lati@s, Mega Mane, MegaZam, Raikou, Starmie, Crocune, Kingdra, Alomomola, Mega Amphy, MegaDoom, and Zapdos. That's just the S, A, and B ranks too. A fully Specially Defensive set I think would just be overall more useful, reliably taking care of much more threats and still being able to sweep. The power drop does mean you need to set up more DDs that with max HP max Atk, but the SpD investment with the insanely good typing and great bulk more than makes up for that imo since it will just give you so many opportunities to set up as you check, counter, or even hard counter a ridiculous amount of OU mons.