Just some quotes from the official website:
With Mega Absol's increased Speed, the Night Slash move with its high critical-hit chance becomes even more potent.
Speed based critical chances return, or just a poorly worded way of saying that Absol needed some more speed?
With the Huge Power Ability, Mega Mawile will wield the Iron Head move with fearsome power!
Iron head, eh? If it's the same base attack stat, well, I'm not too worried.
252+ Atk Choice Band Pure Power Medicham Hi Jump Kick vs. 252 HP / 236+ Def Eviolite Shelgon: 207-244 (61.97 - 73.05%) -- guaranteed 2HKO
252+ Atk Choice Band Huge Power Mawile Iron Head vs. 252 HP / 236+ Def Eviolite Shelgon: 157-186 (47 - 55.68%) -- 75% chance to 2HKO
Such is the fate of good 'mons with bad moves.
Mewtwo's Sp. Atk is what particularly sets this Pokemon apart, placing it as one of the highest among all Pokemon.
With the extra focus provided through Mega Evolution, Mega Mewtwo's Sp. Atk is sure to overwhelm other Pokemon!
It will probably be unstoppable!
Mewtwo: King of the playground since forever. On the other hand, even 255 base SpA doesn't make it the hardest hitting 'mon on turn 1 in Ubers.
252+ SpA Choice Specs Kyogre Water Spout vs. 4 HP / 252+ SpD Blissey in rain: 393-463 (60.27 - 71.01%) -- guaranteed 2HKO
252+ SpA Choice Specs Mewtwo Psychic vs. 4 HP / 252+ SpD Blissey: 240-283 (36.8 - 43.4%) -- 99.76% chance to 3HKO
Ignoring that Mewtwo could just use Psystrike and obliterate said Blissey.
252+ SpA Choice Specs Mewtwo Psystrike vs. 4 HP / 252 Def Blissey: 904-1065 (138.65 - 163.34%) -- guaranteed OHKO