I'm not so sure its balanced... at least compared with other inflaters.
First off, the shuriken is providing up to +7 attack. Second, this is simultaneously going to every copy of the ninja in play.
With one or two in play, this may be balanced (although probably still strong considering how fast
quanta can ramp up in a good deck).
With multiple in play, this could end up quite OP.
Its the overall combination thats OP though I think. If it only affected a single warrior, it would probably be doable, or if it only provided a small bonus, then that might be more balanceable (although this has the same issues as ATK swarm potentially)
I like the +1|-1 suggestion though. Its a fixed value, and ensures that no ninja can grow indefinitely without some kind of outside HP buffing. That forces a duo which would help a lot.