High Attack & Defense
Low Speed & Special Attack
Tyranitar is a good offensive/defensive Pokemon. If you already have both a offensive and defensive Pokemon, don't use M-Tyranitar. You could have added Quick Claw or Choice Scarf, but oh- look! The Tyrantarite blocks the way! What do you do? Don't mega evolve? Or be a slow Tyranitar?
Attack & Special Attack are equal
Defense & Special Defense are about the same; Defense is higher
Speed is horrible (base 20)
You're better off using the Prankster Moveset with normal Sableye.
In my opinion, stick with one mega; not two. M-Kangaskhan is already good; with its Parental Bond ability. The first hit full power, second hit 50%. That defeats all Focus Sash and Sturdy holders/abilities. Power-Up Punch is good for M-Kangaskhan. M-Kangaskhan is so good, it's in Ubers! It's special attack is low, but Attack, Defense, HP, Speed, and Special Defense are okay. Better than M-Tyranitar. M-Kangaskhan only has one weakness: Fighting. M-Tyranitar has six weaknesses; with Fighting being a 4x weakness. Mega Sableye has one weakness, but Fairy types are roaming around a lot these days, such as Sylveon (OU).
I'd say a regular Sableye is better then M-Sableye. With the Prankster Moveset, it's fun to troll with :D