SB mewtwo worth power up now?

I mean if you're planning to short-man M2 and every bit of damage counts, then he's one of the best attackers even with 5Atk.

And as a generalist, M2 with SB is pretty great even with 5A. And it's legacy. Given that you already have one with SB and assuming that one is better, though, leave this current one as a longer-term project. You may get a better one through a trade anyway.

He should be meta relevant for a long time. But then again, same thing goes for the rest of your team.
I'd say save your rare candies and think about it cause it's too expensive

That's so heart breaking... I probably wouldn't, but the truth is that it is still a very powerful attacker even with low Attack IV. If you are obsessive about hitting breakpoints and planning to the millimetre solo raids than it will hurt you see the charts with Psycho Cut compared to non-traded Mewtwo, but otherwise Shadow Ball will deal a huge amount of damage compared to any other mon.

Considering those 4 Tyranitars are worse than useless if Mewtwo has Focus Blast, yeah, it's probably worth it to power up that Attk 5 Mewtwo.

Not a great situation to be in, though. I'd probably trade with others for high-level Scizor or Pinsir if they have them rather than power up that Mewtwo and "waste" all those Candies.