In the battle on Pokemon Smash, Kyurem nearly OHKOed Reshiram with Resh hanging on by what seemed like a pixel. 
So I did some calcs (All at Level 50 like in Pokemon Smash with Neutral Nature Kyurem)
0 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 57.14% - 67.43%
0 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 62.29% - 74.29% (-Def Nature)
0 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 79.38% - 93.75% (0 HP & 0 Def IVs + -Def Nature)
Now I'll give Kyurem Base 150 Attack like Speculated:
0 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 64% - 76%
0 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 70.86% - 84% (-Def Nature)
0 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 90% - 105.62% (0 HP & 0 Def IVs + -Def Nature)
Now back to Base 130 Attack, but let's assume it has an Ability that boosts it's stats:
0 +1 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 84.57% - 100%
0 +1 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 94.29% - 111.43% (-Def Nature)
0 +1 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 118.12% - 139.38% (0 HP & 0 Def IVs + -Def Nature)
Normal Adamant Kyurem vs Hardy Reshiram:
0 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 62.29% - 73.71%
0 Atk Kyurem Freeze Shock vs 0 HP/0 Def Reshiram: 77.5% - 91.88% (0 HP & 0 Def IVs)
I think we can safely say that, Kyurem not getting some form of power-buff is entirely out of the question since those calcs were all 31 Attack IVs Kyurem and even with Adamant & and -Def Nature Target, Kyurem couldn't come close to the damage Black Kyurem did to Reshiram even if he hit for Max Damage.
Other than that, these calcs prove nothing. In fact, maybe it was EVed. Who knows? My statement could be entirely wrong if it was EVed.