Sucker Punch will be a more important move than ever before because Psychic tyes will have a big and natural advantage here. Mega Beedril will be very useful probably.
Mega Beedrill is just a less bulky, slower, and less powerful version of Mega Alakazam with a worse offensive typing. Aside from hitting Clefable and revenge killing Mega Mewtwo Y there isn't much it's going to do that Alakazam just doesn't do better, Bug is one of the worst offensive typings and Beedrill lost its Adaptability. Mewtwo can also dodge the super effective bug hit by mega evolving into Mega Mewtwo X and then just hitting Beedrill with any physical move and it'll go down. The thing is, if you can only mega evolve one pokémon in your battle, why would you pick Beedrill for that over Alakazam or Mewtwo?
The difference between base 145 speed and base 150 speed is relevant, it's a region in which some Dragon Dance pokémon like Mega Gyarados and Dragonite are going to be, meaning that Alakazam outspeeds them after Dragon Dance while Beedrill does not.
Sucker Punch I don't see having much usage either unless it gets more widely distributed. Kangaskhan is the only one even remotely viable that gets it the way it looks now, the others are Raticate, Dugtrio, and Gengar. Gengar should stick with special attacks, Dugtrio's bulk is abysmal and it doesn't hit that hard, and Raticate is just... Raticate, the Alolan one has a base attack stat of 71 and the Kanto one of 81, it's just not hitting hard enough to be scary. Sucker Punch isn't a powerful hit, Kangaskhan could get away with it because of Parental Bond, and some others like Bisharp because of STAB, but in this lineup it's just not scary. To show what I mean:
0 Atk Mega Gengar Sucker Punch vs. 4 HP / 0 Def Mega Mewtwo Y: 52-62 (28.5 - 34%) -- 1.1% chance to 3HKO
Mega Mewtwo Y loses physical bulk when mega evolving, it's not that bulky on the physical side, and it can just stall it out indefinitely if it has Recover. Alolan Raticate is a bit scarier but still hardly a threat.
252 Atk Raticate-Alola Sucker Punch vs. 4 HP / 0 Def Mega Mewtwo Y: 110-132 (60.4 - 72.5%) -- guaranteed 2HKO
That's still far from OHKOing it. Meanwhile, remember that Mewtwo can fight back:
252 SpAtk Mega Mewtwo Y Aura Sphere vs. 0 HP / 0 SpDef Raticate-Alola: 296-352 (197.3 - 234.6%) -- guaranteed OHKO