I can’t be alone by thinking M Manentric EX will not be able to work on its own (Or with Mew). Here’s why
1: Its weak to fighting. Not a big reason, but it certainly doesn’t help with Donphan and Landorus/Lucario still being playable when PHF is released.
1b: Mew EX is weak to Psychic, not good when a lot of Gengar EXs and Mewtwos will be running around.
2: You really rely on that Spirit Link card to get set up. If you whiff it for even one turn, you will be forced to evolve abiding to the Mega Evolution rule, which limits how much you can do in that turn.
2b: This isn’t helped by the fact that if a Seismetoad EX gets an item lock on before you can get your Spirit Link onto Manentic EX, you will again have to still abide by the mega evolution rule, slowing you down.
3: 110 damage does 2HKO the entire of the playable metagame, but top decks have the option to get 1HKOs using something like G Booster or Yveltal EX.
4: Its 2 roles can be easily fufilled by other things, If your using it to power up Colourless attackers, then why aren’t you using DCE? Yes, Aegislash EX will cause problems but that is one matchup out of many. If you need an electric attacker, then why aren’t you running Raichu or Dedenne? These options take up less space in your deck, and have proved to be effective in the past.
So yes, it will be good, but i don’t think it will be good enough to be tier 1