All of this could be obtained with a single Swords Dance after fishing for a free turn and without losing the rest of your team.
I don't agree at all. Not saying the ability will be broken, but this is just not it.
There's no such thing as a free swords dance. Swords Dance needs a turn in a turn based game, and everything that doesn't need a turn to produce the outcome of a turn is already breaking the economy of a turn-based system game (great example: regenerator).
This thing, as Ditto, can potentially become a threat by just entering the field, without support nor previous planification. We are talkinga bout a mon with more than 400 atk, than can hold a LO, cs, cb, whatever you want, that gets periodically stronger obtaining a black glassse boost after you miss 2 of your mons, in what probably are HO or offense teams (so, they lose mons, like a lot).
In fact, the same set you are suggesting, a Swords Dancing one, could benefit from a random 10, 20, 30 or 40% increased damage PLUS the swords dance if you decide to do so plus the item you have equiped. A mon with this attack stat and a average speed that can consistenly get a boost from 20 to 30% in its attack (which effectively is a free LO, not counting the item you've equiped) is nothing to ignore.