Gengar power up?

Yes! Simple answer - best moveset for Gengar, and a useful mon when you need a glass cannon.

I might not go to L35 - it is a niche mon, not one you will use everyday. But I'd definately take it to 30 and look at the breakpoints for raid bosses I plan on using it against to decide if I wanted to power it up any more.

short answer - yep

longer answer - yeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeep

Same answer: yes.
Before there were raids I used it to smack Vaporeons, and later I used it in Machamp, Gengar and Alakazam Raids. Mine is lvl35 btw.
I don't use it as much lately, though.

Only if you have a use for it. I have a 13a/15d/15s Gengar with Shadow Claw/Shadow Ball that I have had at 30 for a long time with only occasional use. My EX Mewtwo raid on Tuesday had Psycho Cut/Focus Blast so I dumped some stardust into Gengar to bring it up a few levels. Not sure if it was worth it for just this one fight, but it still performed admirably and helped deal a lot of damage.

Yes, definitely, it just worths.

Great DPS against several tier 3 raids and seems to be a great (DPS) option against following tier 5 raids that Haven’t been released;
Mew
Celebi
Latias
Latios
Jirachi
Deoxy

I would say a solid investment and a 5/5 future meta relevant Pokémon.

Only thing that I think should stop you would be dust, since you probably are looking to invest a minimum of 150K. Its probably not most DPS for the buck. but if you got dust go for it.

I also had a L20 or below SC/SB 96IV hidden away. It is now L33 and on its way up. It’s pretty awesome. (battle, faint, power mini-revive, potions over and over to get there.)

If you have a use for it. That is mainly just soloing Alakazam and Gengar, where it does shine. Given the amazing IV and valuable legacy moveset, I would power it up as a trophy anyway.