The power of the Commonwealth is already ludicrously strong. In observer games or games where the player doesn't meddle in Eastern Europe, Prussia can never really beat the Commonwealth, and the Commonwealth is so big it becomes the France of the east, blobbing in every direction.
Historically, it was a dysfunctional government that makes it hard for Poland to conduct any unified policy, war or anything, due to the nobles having more power than the king (but the nobles didn't form an effective parliament like England). It was highly decentralized, making what should be a large powerful country weak and unstable, and prey to foreign influence.
If anything, EU IV was very generous with the Commonwealth, giving it some magical, USA-style amazing government full of bonuses, which in reality it was worse than feudal monarchies.
In the player's hand's, it's TOO easy to beat Russia or the Ottomans. Even in the AI's hands, they do a great job keeping the Russians at bay,doing far better than history. Remember, the Commonwealth ceases to exist after 1795 in history, and had become a weak pawn of Russia well before that.