20x is too weak to be all that scary, unless you're using a Dragon deck. Especially if Aromatisse sees no reprint and Xerneas loses good accel / healing. (XY surely rotates before this becomes Standard legal.) Without Aromatisse, Xerneas is stuck both as your accelerator and attacker, which won't last long.
With so much energy on the field, all it takes is a Lysandre to pick off something with a high amount and the attack becomes dinky. I just don't see it being used as the main attacker in a [Y] deck.