Should i PowerUp this Mewtwo or wait?

I've got to catch one tomorrow so will likely be faced with the same dilemma as you. At least 86% is above average.

If you think it's not worth maxing out, you could take it up to level 30, it's probably the most versatile attacker in the game.

By the time you get another Mewtwo raid you'll have enough candies to power that one from 20 to 40, you're close anyway - it's 248 candies to go from level 20 to 40 and 66 to go from level 20 to 30.

if you have this amount of rare candies, you can power up this and the next caught, i have power up my 88,9% today

How much difference is there between 11 and 15 attack? I have a 96% Mewtwo that is 13/15/15. Should I per up to lv 30 or wait for a possibly better one in attack?