Would you power up a 42A/-2D/95S Alakazam?

Yes if it actually was an Alakazam.
As for the real-life Mewtwo it represents...

Yes, unless you have better Mewtwos, in which case power those up first.

Mewtwo with 10/10/10 is a beast. Extra IV make it a little better. And you can always grind a little more dust and rare candy to make up for it if you ever regret the power-up.

No, because your 42A/-2D/95S Alakazam can't get Future Sight.

73% at 30 12a Sept
76% at 30 13a Dec
82% at 30 13a Jan
91% at 34...15a Feb 17th
I need 134 more rare candies to get to 40.

More than 50% chance to get a better one if you get another invite, and they seem to be a lot easier to get nowadays. Or power it up to just 30. Fact is it's less than 2% worse than a perfect Mewtwo, but since the resource cost is high, you should only rush to max it out if it's very unlikely you'll see a better one.