Well, power is relative when it can be beaten by Gengar, but the new boss patched into arcade versions of the Pokémon fighting game is definitely a darker version of Mewtwo. Appearing in video footage on Japan's NicoNico service (via NeoGAF), the boss fans have dubbed Dark Mewtwo looks a bit like Mega Mewtwo X with extra bits. Here's a video of the new Mewtwo versus Gengar grabbed from NicoNico by YouTube's TrackerTDGaming.
UPDATE: Here's the Shadow Mewtwo reveal video from the official Pokémon channel.
Shadow Mewtwo will be joining Pokkén Tournament on Wii U in spring 2016.