All it does, and all the ex raid system does is to incite hatred, envy and rage. For every one of your Mewtwos, there is 100 of these type of people:
"I am level 40. I had every pokemon from bulbasaur to groudon to unown. But because of this shitty ex raid system, i can no longer get a full pokedex and nobody calls me the pokemon master."
To be honest in my opinion doing so means you are a supporter of the EX Raid system and thus makes you the supporter of an organization of swindlers, charlatans and clowns. If you ask which mewtwo you should power up or should you power up a mewtwo in ANY board, this should be the same.
Do not spend a dime until they fix this system, and issue a refund if you do. Also if you have a Mewtwo keep it secret and don't let anyone know it. For god's sake. If you brag about your mewtwo and ask that should you power up a 75%, you only shown how disgusting you are.
The answer for mewtwo power ups is very simple and is a no brainer. Power it up. You have no idea if you can get a better one or another one at all, and if it has hyper beam, charge tm it. It is a decent and powerful pokemon.
Please don't get tricked into beliving any fix for the "EX RAID System" coming soon, my redeem is to scrap it at all and issue mewtwo for regular tier 5 raids for 2 weeks then between breaks. (In other words, If there are no new legendary raids, the tier 5 should be Mewtwo.)
Thank you. Sorry for my salt fries but i really have to do it. So far i had 106 raids in a month starting from the beginning of my journey at mm12/dd3/yy2017 and i didn't get an ex raid pass, Even when multiple eligible gyms are raided, While people who did a single magikarp got it and others got multiple. To make it worse, sponsored gyms do not exist in HK and the only way to get one is to gamble at parks. If i make 1000HKD for each rare pokemon ran after a few Gold Razz Ultra Curveball Great i would had been made millions, if 20000HKD for each ex raid pass i would make 0.