Well, based on what I've been reading lately about X and Y's Mega Evolution, I'm thinking the stat changes in battle are not specific numbers, but stages, rather.
Like I've heard that Mawile's Defense and Special Defense each raise sharply (maybe two stages?) when it evolves in Mega Mawile. That's very impressive.
Also, some Mega Pokemon gain new Abilities, following my example of Mega Mawile, it has the Ability Huge Power. How's that for destructive?
Atk, Def, and Sp. Def increase? Sounds awesome to me. Mawile was my favorite Gen 3 Pokemon, so I'm so excited for that!