Thinking about M Lucario, I wonder how good the move really is.
Against Virgen, it could be really good if you could copy G Booster. Though if not, you still have Megalo Cannon which is solid enough as it is.
Against fighting decks, you have quite a few moves to choose from, but you don't get all the damage boosts that make fighting decks work. You also couldn't use Landorus's big attack unless you run fighting energy. At least Gengar resists fighting and Lucario has psychic weakness.
Similar problem with plasma decks; much of the power comes from Deoxys EX, who's attack you wouldn't be able to copy unless you happen to run plasma energy (same thing with Lugia). You could copy thundurus's second attack (solid 90) either of Kyurems attacks, or Genesect, if they happen to be running that.
Gengar's weak to dark so it probably wouldn't be too good against Yveltal decks, but there are plenty of good attacks to copy from Evil Ball to Y Cyclone, to Night Spear.
Pyroar: Gengar can counter Pyroar just fine (not a basic), but hopefully an opponent has other attackers you could copy to deal with it (Mewtwo and Charizard come to mind).
Blastoise: Probably the best matchup if they're running BKEX. If you run the stadium, you can kill pretty much anything for just 2 energy (tried this with Foul Play Zoroark on PTCGO, could still use it even though I didn't have 3 energy to discard). Though if it's just Blastoise/Keldeo, have fun doing 50/60 damage.
So what I'm getting at is that most of the attacks you could copy are already cheaper than the attack cost of Gengar, and don't require a mega evolution. The ones you would want to use require some kind of specific setup. I don't think M Gengar is bad; I think it could be good against decks that use a variety of attackers (like fighting or plasma, even though they don't get the same benefits), and also cards like BKEX, which are designed to OHKO, but have funky attack costs (making it more practical to use with Gengar).