Should I power up my 89% Mewtwo?

The stats you've listed give it 91% IVs, not 89%. I think that's a very respectable Mewtwo (esp. with 14 Attack) and worth powering up. No one really knows how frequent Mewtwo raids will be in the future at this point, but no one really knows when Mewtwo will be released, either. You could have 2 months before your next EX raid; do you want to wait that long for the possibility of getting a better one?

Ladies and Gentlemen, now presenting the humblebrag!

I maxed out my Mewtwo that is way worse with 84% and 11 attack- best stardust investment the amount of candies I've reviewed from Alakazam, Machamp, Gengar raids and dust along with enjoyment of using a new mon way outweighs waiting hoping to get another invite and hoping it's slightly better it could be months for all we know imo but enjoy the game how you see fit

It blows my mind that people would even think twice about powering up mew two. He is so OP that his IV don't even matter. Mine has confusion focus blast. If you're such a pokesnob that you will only power up stuff that is perfect or near perfect i have only to say, sheeeesh!!! So if you are lucky enough to have one power it up as high as you can. Gyms are soooo easy to defeat anyways, does 100 cp make a difference on a mew two?

Well, it's up to you, in my city, my neighborhood group is very well organized and we got Mewtwo Ex Raid instead of the downtown gyms or the gyms located at the university.

I got a 82%, 15 atk IV Mewtwo, and didn't powered up him, I know that I could power him up as his atk IV is the best it can get, but I don't feel like it.

BUT, putting a ~4K CP Mewtwo as the first pokemon in any raid that is not happening on my neighborhood has such a wow factor

No you shouldnt

Yeah, either those stats aren't right, or yours is 91%. It's a legitimate question though, and I've been wondering the same thing about mine, which actually is 89% (14 HP, 13 Attack, 13 Defense). I'm tempted to wait until I get one with 15 attack, but I don't know when that will be. On the other hand, 89% is pretty good. My son's was only 80%. Does attack really matter that much?

My only Mewtwo is similar: 13 attack, 13 defense, 15 stamina. Currently powered up to level 35 and destroying Ttars and Blisseys (Psycho Cut / Focus Blast). I say power away!! How many other mons do you have with a base attack of 300?