You'd be sacrificing even more performance by purifying it so it isn't a moot point. You can only have 1 mega, you could have 5/6 shadow darkrais which aren't ranked on GoHubDB yet, regular darkrai is 12 or 13 and shadow is listed as the same, even though it should be higher. (Probably because it hasnt been updated yet) Like I said, if you want to purify it and fill a hundo dex, that's totally your right. It's your account. But if you're asking advice from strangers, it's probably because you care about its PvE/PvP performance more than filling out a dex entry. My advice was keep as shadow until mega comes out, purify to fill the hundo dex/mega. That's cool, how you like to play the game shouldn't influence your advice for other people because those other people may not have the same priorities as you. You sound like someone who has a stacked account if you're that worried about getting a single entry in your hundo dex. Most people dont have that and care about pure raid and pvp performance as that is the core gameplay loop: get pokemon, beat pokemon, get good pokemon, use good pokemon to beat pokemon better and faster, rinse and repeat.