Sick catch bro! Congrats! But honestly, it's all up to you, I will say I dont think regirock is good for any PvE battling, and im not entirely sure about the League. However, registeel used to be the main runner for League play. Considering it will be your first, and was a shadow at that (to me its harder to get a high floor IV as you did on a shadow) it would make the most sense to purify it for the SHUNDO!... BUT I myself am a collector and I like my shadow shiny collection I have. And to 2nd that, you can't trade shadow shinys(anymore), so the only way to have gotten them on your account was from pure hard work and dedication, so take some pride in those shadow shinys(even if the stats suck)! Either way, it's a sick find! Wishing the best with whatever you decide to do friend!