I think Shadow Claw Gengar is worth maxing since it's so dependent on HP and defensive breakpoints. Hex/Shadow Ball is decent for Alakazams with PC but that's about it. It would be better to evolve a weather-boosted low IV one. Save the powering up for legendaries, TTars (Also good for Alakazam Raids) and for the Shadow Claw one.
Gengar question
I just evolved a L27, 15/12/14 weather boosted Gastly that I just caught, on the spot after checking its IVs.
The Gengar got Sucker Punch/Shadow Ball. Shadow Ball is a no-brainer, but should I use a fast TM?
How much should I power the Gengar to? How useful will this Gengar be for Gengar/Alakazam raids without Shadow Claw? I'm aiming to solo a PC Alakazam.
I only have 1 high level Gengar with Shadow Ball now, and he is my only one with SC.
Gastly isn't very common here outside of events, and I've exhausted almost all of my candy evolving high level, decent IV Gastlys, and powering up my legacy SC/ShB Gengar to L34.5 (defensive Mewtwo breakpoint).
Answers
I would also be hesitant to spend resources on a lev 27 Gengar, and rather wait for a higher level one, but you have already spent a lot of candy by evolving it to Gengar so it is a little tricky. Where I live we have often cloudy weather and an abundance of Gengars. Yesterday I was lucky to catch a lev 34 Haunter with high IV that I will evolve, but also a decent lev 30 Gastly that I sent to the professor.
It's fine for PC Alakazam, needs one power up to also work against C Alakazam (able to tank 3 Confusions and thus fire off 2 Shadow Balls).
It's very useful for Alakazam/Gengar raids. Not counting Mewtwo, nothing challenges SB Gengar's DPS against those raids. It's glass, but the DPS is insane. It's fine without an FTM, may even perform better in some cases, but Hex is generally better so I would change it if you have TMs.