Power up or not?

ShB or SlB?
14 attack at 50% IV
or 14 attack at above 82% ?

If its Shadow Ball, and overall IVs are good.

If tou need/want a good gengar, why not
If you are lacking dust or feel there are more valuable things you could spend it on, then save it for abit.

It isn't essential, imo, I'm at 8mil xp having never powered up a Gengar, Flareon, or Jolteon--too fragile for me. I'm not a gamer in the Last Starfighter sense, I won't put resources into something that'll get zapped so easily after one mistake

If you have the stardust to spare than I'd say go for it.

I have a powered up Shadow Claw/Shadow Ball Gengar I use very often. He's my main 'sweeper' (Dragonite is my other) that I use the third time through a gym to mop up the low motivation defenders as quickly as possible. He's perfect at this role (but just don't use him against normal types like Blissey/Snorlax)