Well personally I don't see anything wrong with it. It's 'Good Enough' as you say :P
Since you're playing X, you'll get Mewtwo-X if you try megaevolving, which is the physical variant - so having a perfect attack IV is excellent. Even though it has good special attack anyway, and Adamant takes away from that, it still adds to Attack so it's a decent nature. High speed is always nice, and Special Defense is a bonus. Mewtwo is a sweeper, so defensive stats don't matter as much to it, so you're good with what you have I'd say.