Mewtwo

Power it up. Shadow ball is mewtwo's best move, ant the attack stat is all that really matters, even though IV's as a whole dont matter as much as people make them out to be.

Depends. If you have the resources and you're not likely to get another in a trade sometime soon, for it. Shadow Ball is legacy, as well as an excellent move on a powerful pokemon, and it performs well in Master League. Not to mention that you'll have opportunity to use it in the upcoming raids against Gen IV psychic type raid bosses, even though it got a relative nerf with the SE multiplier increase.

TL;DR: Bring it up!

If that's the only SB Mewtwo you got, hells yeah you should power it up to lvl.40 asayc.
If you have any better, I suppose you'll never need more than three. Right now I am leveling up my third.
Without my hundo SB Mewtwo I would be freaking out when attacking gyms. It is an excellent attacking core when paired up to other beasts like Machamp and Metagross.

I'd say yes. I've maxed two SB mewtwo and have two more ready to go that I'll likely get to eventually.

Admittedly mine all have better IVs, but they don't make a huge difference. Especially when you have 14 attack iv - you'll hit almost all the breakpoints. My next maxing project for Mewtwo will be a lucky 11 attack SB from before the lucky rework (then finally a regular 14 attack one, both of those 87%).