The changes to Mewtwo's up air are pretty danged significant, as far as I can tell. Basically the move went from a minor nuisance to something that can potentially reverse the momentum of a match.
First, even putting aside its kill power now (especially in high-altitude juggles), consider a situation where Mewtwo is recovering off stage and goes below the ledge. In the past, Mewtwo could up air through the platform to hit the opponent. It was a cute move, but it didn't do all that much, and sometimes you could also get hit out of it and fall to your death.
Now, though, not only are the hitboxes improved, but the increased knockback means that if the opponent gets hit by your tail they get sent rocketing through the air. It might not be enough to kill them, no, but it gives you ample time to get back on stage and now you're on stable ground, they're above you in a disadvantageous state, AND they have to be wary about continued up air juggles.
Second, because it IS a kill move now at higher percents, it's another option to threaten with while landing. In the past you could maybe get away with landing a fair or a dair on top of unsuspecting opponents, but that was only one of two offensive measures. However, up air is now an option to mix up that situation (as is bair and its new fancy power/hitboxes).
I don't know if Mewtwo's going to shoot up the tier lists or anything, but he's basically gotten a new significant tool on top of a bunch of smaller buffs (those landing lag buffs my goodness), and it means that his advantage, disadvantage, and neutral are all better in a noticeable way.