I don't wish to make any kind of extended debate here because it would be weak speculation and loudmouthing, but I still find it a bit interesting to discuss this so ill proceed. I want to establish some things for dicussion here.
1) Stop comparing Life Orb attackers to Mega Attackers. This was something that plagued discussion pre-XY, and I'm a bit frustrated it's still being bought into. The recoil that a life orb incurs is catastrophic in the cases of trying to preserve health to survive weaker priority, as well as residual damage coupled with damage taken on set up. Mega SAlamence already exceeds life orb Sally in power when using its Flying STAB, and isntead of losing HP it gains a lot of defense. This disparity here makes comparisons between the two rather wrongful, where Life Orb Salamence would fall to a Hippowdon's Ice Fang after Life Orb damage, and such and such, Mega Salamence kinda laughs at it. Look no further than Mega Gyarados, and Mega Tyranitar who are much weaker than their Life Orb counterparts, but for reasons similar to Salamence are far, far superior Dragon Dance attackers.
projecting 120 defense: 252 Atk Mamoswine Ice Shard vs. 0 HP / 0 Def Salamence: 228-268 (68.8 - 80.9%) -- guaranteed 2HKO.
Damn straight. Extrapolating, the lack of Life Orb lets SAlamence stand up to Brave Birds, Sucker Punches, Bullet Punches, and countless other priority attacks that it previously could not.
2) Life Orb Dragon Dancers have always been unpoluar anyways. This is because... the residual damage and lack of set up opportunity meant surviability items like Lum Berry or LEftovers were far better at doing the job of cleaning house, sweeping, if you will. Life Orbs are reserved for all out attackers, or Wallbreaking boosters that have a hole punching, or hit and run style of attacking that either keeps them out of danger, or sees it fit to let them be KO'd to remove the defensive keystone. Obviously Swords Dance > Dragon Dance for a wallbreaker. Because of this comparing LO sally to MEga Salamence is pretty misguided, because, yeah actually LO Salamence would destroy offensive teams like I project Mega Salamence will be able to, if it could stand up to some weak priority or attacks on set up without keeling over, but it cant, and mega Sally can.
Life Orb Salamence is as strong as Mega charizard X yet its a piece of crap in comparison. OFfensive calculations while ignoring the massive defensive and speed upgrade ignores an extremely important part of the game.
It's also really going to have a bit of a problem covering all its bases, being either hard walled by Heatran (if it doesn't bring Earthquake to the party), Skarmory (if it doesn't bring Fire Blast), Mega Diancie (if it doesn't have steel wing), etc. Like 9 tales said, as well, unless it gets a ridiculously high attack buff, none of its attacks bar Return/ Double Edge/ Facade are going to hit as hard as its previous LO sets.
A simple Return / Dragon Dance / Earthquake / Fire Blast set is sufficient for almost the entire metgame. Outrage can be used to muscle past Rotom-W if it needs to. Diance is easily dealt with with Earthquake, even if it does not OHKO (It probably does to all but physically defensive Diancie), Mega Salamence can probably live the Diamond Storm