I mean, 170 HP is immense but Mewtwo EX does indeed have most excellent Base Stats in the video games, from which the TCG is derived. Of course the TCG is neither capable of perfectly simulating the video games, nor is it desirable to do so, but the source material is the source material; deviations should happen primarily because they must.
Mewtwo in the video games has above average Base HP, Defense, and Special Defense scores. Unfortunately the TCG lacks a good way to represent Defense and Special Defense scores, so unless you want any Pokemon with an above/below average Defense or Special Defense score to have an Ability or Attack that reflects it, the logical solution is to factor those scores into HP.
I labored under the mistaken notion that with respect to HP scores, Stage 2 > Stage 1 > Basic Pokemon. This was faulty. Simply look at those Base Stats from the video games to get an idea of how durable the Pokemon is supposed to be, as a general guide. So Mewtwo EX has a TCG HP score that reflexes those video game scores. Given the damage output of so many Pokemon, as asinine as it may sound it would be older cards that possess too low an HP score.
Mewtwo EX's attacks are also justified by its attack/special attack scores. The fact that it is a Pokemon EX is an important caution to prevent it from being too powerful (since as a pseudo-Legendary Pokemon, it actually is overpowered in the video games). Without help, it doesn't even OHKO the most popular Basic attacker using its "big" attack, Psydrive. So where is the issue?
X-Ball, except X-Ball is also pretty reasonable giving the card and the metagame... except for two overarching problems that have plagued the current format; fast Energy acceleration and an overly vulnerable Bench.
Imagine if we lacked Double Colorless Energy and if Pokemon like Emboar, Typhlosion, and, and Eelektrik weren't pushing the boundaries of other forms of Energy attachment. Now it isn't almost a "given" that any Mewtwo drops and can use X-Ball; it needs a turn of prep or a more balanced form of Energy acceleration: Electrode Prime, Exp. Share, or Gardevoir. Electrode works for anything, but hey it costs you a Prize! Exp. Share requires you have Mewtwo EX in play with it equipped (instead of Eviolite) and that you had an Active Pokemon with an Energy get KOed by your opponent's Pokemon's attack. Gardevoir works for Psychic Pokemon using Psychic-Type Energy, and I have no problem with a Stage 2 Pokemon being required to maximize a Pokemon EX's performance.
Emboar and Typhlosion aren't big deals now, but they probably would be if Eelektrik either couldn't attach to non-{L} Pokemon or had an appropriate penalty. I don't think we'd be much happier in a format subbing either Typhlosion Prime or Emboar decks in place of Eelektrik decks. Typhlosion Prime almost gets it right, except again the Energy attachment is working on anything, and with HP inflation a single damage counter means very little to most attackers. Even a plain Reshiram doesn't mind getting a double attachment from Typhlosion; an Eviolite will still leave it resilient enough to survive a shot that does 120 points of damage.
Mewtwo Wars happen because right now, any deck (but especially something built around non-Pokemon-Type specific Energy acceleration like Eelektrik) can plunk one down, fully powered for X-Ball in a single turn. Deny Mewtwo EX this option, and suddenly it is on par with other Pokemon EX. Would you risk Benching a Mewtwo EX to manually power-it up over the course of two turns? Even knowing that it is a probable target for Pokemon Catcher followed by a less generic big Pokemon-EX attacker?
Pokemon Catcher is what makes it so hard to set-up the Stage 2 Pokemon that could otherwise go toe-to-toe (when they have toes, anyway) with Pokemon EX. I believe Bench disruption needs to be widely available, possibly generic, but Pokemon Catcher is far too powerful. The only reason it is an improvement is we already had Pokemon Reversal, a card I didn't realize was also unbalanced until this format; when Pokemon Reversal is the best option for Bench disruption, we fall prey to another round of "Flippymon".
People are willing to chance that their Stage 2 decks can work because they are counting on most opponent's not getting enough "heads"... but that's still hoping for luck. Considering Pokemon Reversal already had whittled down the amount of viable Stage 2 Pokemon before Pokemon Catcher debuted, it wasn't even that much hope.
If neither existed, I would want a suitable replacement, preferably a Supporter or Energy. I don't want to make it a Pokemon, since that just makes a "staple" Pokemon clogging decks. We saw what that was like last format when we had a lot of Pokemon that, even with coming-into-play-effects were still spammed. A Supporter would need to be a more powerful Pokemon Catcher, I think. Not too much more, mind you. An Energy would need to be a step up from the old Cyclone Energy; you need to be able to force up something specific. The difference in either place is you're giving up something pretty valuable, and neither card is easy to recycle.