I haven't taken a look at the official ORAS website before today, and now I wish I hadn't. On top of confirming stat boosts for Primal Kyogre and Groudon just like for mega evolutions (Kyogre's special attack and Groudon's physical attack are mentioned explicitly), you can read this tidbit in Primal Groudon's section:
And it appears that its Ability might get a boost!
Yeah, depressing, I know. We all know what that means, and we all saw it coming. Kyogre's almost definitely getting the same treatment, too. Honestly, I could've seen perma-weather Primal Kyogre/Groudon getting used a lot even WITHOUT stat boosts. They're really not necessary, but I guess Gamefreak is just THAT sick of Mega Mewtwo Y.
Also, last month I mentioned a suspect test aimed at stopping the superdominance of full Baton Pass teams, of which there existed exactly two: the one with Mr. Mime as last team member, and the one with Zapdos as last team member. And Mr. Mime, despite being absolutely terrible in any other context, ALMOST MADE THE OU CUT-OFF LINE at one point. So the most sensible scenario was approved through suspect voting: no more than three Baton Pass users on a team now. Certainly beats any kind of combo ban with Speed Boost or Magic Bounce or what have you, though said option still had more support than I expected. And it sure beats doing nothing when you can induce a record amount of ragequits just by having Vaporeon, Sylveon, Espeon, Smeargle, Scolipede and either Mr. Mime or Zapdos show up on team preview. I'm not kidding, you could have all six of these, but only have them have Tackle or whatever, and you'd still get a pretty decent win/loss ratio with all the ragequits. I wish someone actually experimented with that, because not even in the heyday of rain teams were the NOPEs this loud.
With that said, if you were a scrub looking for some easy wins with full BP things, think again. Mr. Mime usage works well as a barometer since it's so bad otherwise. Look at where it is in the unweighted stats. #158. Holy crap that's low. Now go into the weighted stats, starting with 1500 as the minimum rating. #137, a slight increase, but nothing insane. Now increase the minimum rating to 1695, which is what's currently used for tiering. #76, way higher than before. And just for that extra kick in the nuts, with a minimum rating of 1825, which represents the top 0.4% players or so, it comes in at #65. So it really was a team for very skilled players, and top players with this team were nigh unstoppable, even by top players who didn't run it. Hence the decision that was taken - and you won't see me complaining.