Power up Gengar w/ legacy shadow claw?

Absolutely. I would kill for a Gengar with that. And its IVs aren't even bad either.

If you want to solo Alakazam then yes.

It's a expensive toy. You'll need to invest alot of dust for a small increase of DPS.

However the small increase is against many upcoming legendary raids and Also useful soloing tier 3.

A Gengar with 0at iv and shadow claw will out perform a Gengar with 15 at iv and Hex. Power him to at least L33, the last break point against Alakazam. You could make the case for maxing him, due to his low base defense and stamina.

I'd power him up to level 30, and try him out in raids. If you like him, power further. I have one that I am still testing out at level 30.

I would and did!

I don´t think it´s worth it, what makes gengar a great attacker against psychic types is his awesome charge move shadow ball + stab. If he has shadow claw or hex or sucker punch, it doesn´t matter, hex and sucker punch load his charge move even faster, although shadow claw gives a little more damage as a fast move.

Soloing alakazam for example, what makes you beat the boss or not with confusion is fire 2 shadow balls instead 1, and to get it, you need a gengar at level 40 with 15iv Def and at least 13iv HP regardless his fast move. A level 40 Gengar with these IVs can take 4 confusions with no dodge and without faint.