15 Attack is extremely useful - I would power it up. That moveset isn't bad either. If you''re gonna change it (Personally I would, but it isn't terrible as is), wait until you get at least one more Charged TM in case you get bad Hyper Beam.
The nice thing about Mewtwo is, because of its insane stats and diverse movepool, powering up more than one is still a good idea, should you come across another one.