I think Megas came about more as a shake up than actual balanceSo far, every Mega has gained a full 100 points to its Base Stat Total.
That bodes ill for the potential utility of Mega Mawile, who would end up with a still-poor 480 if the patterns hold, and whose Huge Power boosted base 85 Attack is being eclipsed by Megas of other, more well-rounded Pokemon.
If Game Freak was trying to use Megas to stimulate balance, they've not done a very good job of it.