Mewtwo Power up

I got 5 so far and yours is better than all of them :D
However, I would still wait. I mean, it is unlikely that Mewtwo will remain eternally as the ex-raid Pokemon. Just be patient until he is changed, since you might still get a far better one.

If you think it will be your only one any time soon, then yes. It's quite a decent Mewtwo even though it isn't a wonder that simply amazes me and can battle with the best of them.

Mewtwo is overrated and pretty fragile, i would wait.
C/SB can't deal with gym (Blis,lax,chan)
PC/P - dps is not much higher than Zam Pc/Fs.

With ATK 13 the number of breakpoints he cannot reach is probably very limited. In example, Mewtwo with Confusion needs at least ATK 12 in order to be able to inflict 33 damage on a Machamp boss in normal weather. So, go ahead and power it up. If you have only one by now (from the glitch weekend?) it might be months before you get a second one. Months during which you could play with your Mewtwo.