Meteor Mash's tentative stats, according to the video, are 90 Power, 2.7 second duration (Dynamic Punch / Avalanche). It's not super broken but that's p good and if they go with this baseline Metagross should be insane.
Here's a document with the full rebalance factored in. Notable things:
1) Everything gets a Defense nerf or no change. The frailer the Pokémon, the closer it was to receiving no change. The bulkier the Pokémon, the more defense it would lose. If a Pokémon specializes in one defensive stat (like Cloyster and Steelix) and not the other, it gets penalized. Balanced Pokémon are rewarded.
2) Mostly everything gets an HP buff, with Pokemon that had lower base HP stats getting massive buffs. Notably, the likes of Dusclops, Shuckle and Deoxys improve substantially.
3) Blissey. Blissey is the Pokémon affected the most by this, as it lost 60 points in Defense and lost some HP. It is much frailer now and, once they switch the formula again, should be easier to manhandle in gyms. That being said, it does decay in gyms less thanks to its 2757 max CP, so that's a tiny buff.
4) Mewtwo. Notably, Mewtwo was NOT changed during this initial release. I speculate that it's because of it's current Tier Boss status, and will likely have its changes applied later on. There is precedence with this happening to Lapras in the November 2016 CP rebalance, as it was a Pokémon that was having a massive event in Japan at the time and also did not receive its changes until later on. Once its changes are applied, Mewtwo is the strongest Legendary in the series currently in Go with an improved HP stat, and should now be above Groudon / Kyogre in sheer CP.
Presumably because of the HP buffs (and nothing being healed to compensate), the formula change was reverted. Should get it before Gen 4 drops alongside our Pokémon being properly healed.