As Zachiel has covered, you don’t get to use a Large two-handed weapon as a two-handed weapon when you’re Medium, even with the −2 penalty. But even if we ignore that and let you,
Wielding an inappropriately-sized weapon is a mathematically bad choice every time.
(barring some kind of feat or class feature that waives the penalty, but even then the damage boost is not worth a feat)
On average, you would trade −2 attack for +1 damage. Larger weapons do somewhat better (1d12 to 3d6 is the best, with ~4 damage boost, but you could have just used a 2d6 weapon to begin with, which makes the boost only ~3). This is not a “lot more damage.”
Note that in most cases, this is massively worse than even one-handed Power Attack, and one-handed Power Attack is not a good idea. Even in the best cases, it’s a very-poor-man’s Power Attack, except it’s not optional, and that’s only in comparison to Pathfinder’s unnecessarily-nerfed Power Attack.
Furthermore, attack bonuses are small, rare, often don’t stack, and are one of the few aspects of the game where the math is reasonably consistent, which effectively means that most characters can’t take very many penalties without severely increasing their miss rate. Power Attack itself is often dubious, and with wielding an inappropriately-sized weapon effectively giving you a Power Attack you can't turn off (without drawing a separate weapon which you have to pay for separately), you would often find yourself completely useless. More often than a typical Pathfinder mundane melee character, I mean.
So no, it is not powerful; it’s quite the opposite of that. It is shooting yourself in the foot for absolutely no reason.
