Heck yeah i'd power it up, 15 atk on a rayquaza's fantastic. I did a trade, nonlucky 93% 13 atk, 15 def, maxed it out (my only Ray at the moment)
with giratina out and likely palkia next, it will be exceedingly valuable and we dont know when rayquaza comes back, and having one extra rayquaza never hurt anyone (other than a raidboss)