If this effect was for one turn, maybe, but permanent via counters!? I made a deck around her and on my first run she became a 19 19 on turn 5. I replaced her on turn 6 and she was a 22 22. Needless to say, my opponent didn't survive to turn 7.

The only other thing I can think of to balance her besides making the effect one-turn only is to add a built-in overload (since Jean often couldn't control her own powers). It could say _At the start of the combat phase, if Jean Grey has more than 6 +1 +1 counters, KO Jean Grey._

shes cool

Very easy combo with Cable. Drop her on turn 5, let a few more of her cards hit the discard, then Bodyslide her out before your attack phase. This gives you more counters without having to spend the resource points to replace her.