After a month of testing, taking into account both our personal experience as players and the community input from this thread, the OU Council has unanimously decided that Mega Salamence should be immediately placed in Ubers.

The ORAS OU ladder and, starting from last week, the Standard OU ladder on Pokémon Showdown have led us to believe that Mega Salamence is simply too much for the OU tier and that its presence is actually hindering the development of the ORAS metagame.

Mega Salamence's unhealthy presence in the metagame can be summed up in 3 points:

1) Incredible physical bulk
By sheer stats, Mega Salamence is one of the sturdiest offensive Pokémon in the game. 95 / 130 / 90 defenses are exceptional for a sweeper and give Mega Salamence the possiblity to set up with impunity on a plethora of defensive and offensive Pokémon. Combine this with Intimidate and the fact that people often opt to invest in a lot of bulk (as Salamence does not need significant Attack investment to sweep against offense at +1) and it becomes immediately clear that both killing M-Salamence on the turn it comes in and revenge killing it after it has fainted an opposing Pokémon is no easy feat. In fact, Mega Salamence's physical defense is comparable to that of Skarmory.

Quote

252 Atk Terrakion Stone Edge vs. 4 HP / 0 Def Salamence: 260-308 (78.3 - 92.7%)
252 Atk Life Orb Mamoswine Ice Shard vs. 4 HP / 0 Def Salamence: 270-328 (81.3 - 98.7%)
252+ Atk Choice Band Talonflame Brave Bird vs. 4 HP / 0 Def Salamence: 187-222 (56.3 - 66.8%)
252+ Atk Life Orb Mold Breaker Excadrill Rock Slide vs. 4 HP / 0 Def Salamence: 192-229 (57.8 - 68.9%)
-1 252 Atk Garchomp Outrage vs. 4 HP / 0 Def Salamence: 210-248 (63.2 - 74.6%)
252 Atk Life Orb Weavile Ice Shard vs. 4 HP / 0 Def Salamence: 265-312 (79.8 - 93.9%)
252+ Atk Choice Band Dragonite Extreme Speed vs. 4 HP / 0 Def Salamence: 118-139 (35.5 - 41.8%)

As the above damage calculations prove, taking down M-Salamence is incredibly hard, even if it opts to not invest in bulk at all.

2) Stupidly powerful offensive presence
Mega Salamence receives a significant boost in both its offensive stats and Speed. With 145 / 120 offenses combined with Aerilate, 120 base Speed and an extremely large movepool, Mega Salamence is one of the most unpredictable and hard hitting sweepers in the game. No Pokémon outside of Eviolite Porygon-2 can safely switch into Mega Salamence.
A few damage calculations will show how easy it is for Mega Salamence to sweep/wallbreak against the vast majority of the OU metagame:

Quote

252 Atk Aerilate Salamence Return vs. 252 HP / 88+ Def Ferrothorn: 159-187 (45.1 - 53.1%)
252 Atk Aerilate Salamence Return vs. 228 HP / 0 Def Azumarill: 283-334 (71.1 - 83.9%)
252 Atk Aerilate Salamence Return vs. 248 HP / 116 Def Mega Scizor: 160-190 (46.6 - 55.3%)
252 Atk Aerilate Salamence Return vs. 252 HP / 0 Def Mew: 237-279 (58.6 - 69%)
252 Atk Aerilate Salamence Return vs. 0 HP / 4 Def Mega Charizard X: 214-253 (72 - 85.1%)
252 Atk Aerilate Salamence Return vs. 0 HP / 4 Def Mega Gyarados: 219-258 (66.1 - 77.9%)
252 Atk Salamence Earthquake vs. 248 HP / 0 Def Tyranitar: 218-258 (54 - 64%)
4 SpA Salamence Fire Blast vs. 252 HP / 4 SpD Skarmory: 248-292 (74.2 - 87.4%)
+1 252 Atk Aerilate Salamence Return vs. 252 HP / 4 Def Hippowdon: 306-360 (72.8 - 85.7%)
+1 252 Atk Aerilate Salamence Return vs. 244 HP / 28+ Def Gliscor: 259-306 (73.5 - 86.9%)
+1 252 Atk Aerilate Salamence Return vs. 0 HP / 0 Def Thundurus: 237-279 (79.2 - 93.3%)
-1 252 Atk Aerilate Salamence Return vs. 252 HP / 0+ Def Landorus-T: 157-186 (41 - 48.6%)

While its most common set (Substitute / Roost / Dragon Dance / Return) can be walled by Skarmory, Heatran, Zapdos and defensive Rotom-W (assuming it has Hidden Power Ice), all of the aforementioned Pokémon can be defeated with the appropriate move. For instance, Mega Salamence can use Fire Blast for Skarmory, Earthquake for Heatran and Outrage/Draco Meteor for Rotom. On top of that, Mega Salamence can run many other sets such as: Sub Dragon Dance 2 attacks, Roost 3 attacks, Dragon Dance 3 attacks, Dragon Dance Roost 2 attacks, Dragon Dance Refresh, fully Special, and so on, which are all viable. While they are not equally as effective as the most common set, this means that there is no surefire counter to Mega Salamence. While the same can be said for other offensive OU powerhouses, none of them have the same combination of offensive presence, staying power and Speed as Mega Salamence. In the end, it's safe to say that Mega Salamence’s presence in the Overused tier simply overshadows the worth of using other Megas.

3) Centralization and negative effects on team building
For the reasons listed in the above two paragraphs, every team has to overprepare to "counter" Mega Salamence. The amount of teams that run obscure and niche counters/revenge killers on the ladder is impressive. People are forced to run defensive Porygon-2, HP Ice Rotom-W, defensive Zapdos and Cresselia in order to deal with the threat that Mega Salamence represents. On top of that, none of these Pokémon are exactly viable on offensive teams, meaning that said teams are either forced to run gimmicks such as Choice Scarf Greninja in order to revenge kill Mega Salamence or to completely twist their structure and sacrifice a Pokémon slot to carry a dedicated answer to it.

While we aren't directly using the 4th Generation suspect characteristics anymore in order to address bans, they still provide a reliable framework to assess the situation. By looking at the Offensive Characteristic:

Quote

A Pokémon is uber if, in common battle conditions, it is capable of sweeping through a significant portion of teams in the metagame with little effort.

it's easy to notice how Mega Salamence perfectly fits this definition.
Nonetheless, Mega Salamence also falls under the Support Characteristic:

Quote

A Pokémon is uber if, in common battle conditions, it can consistently set up a situation in which it makes it substantially easier for other pokemon to sweep.

Considering that it can easily eliminate or severely weaken common defensive cores, this makes it easy for other Pokémon (such as Talonflame) to sweep even teams that are prepared to handle the so called "bird spam" strategies.

For all the reasons listed above, the OU Council has decided to place Mega Salamence in Ubers with immediate effect.