Which legacy move does it have? Sludge Wave and Dark Pulse aren't anything special but Shadow Claw is it's best quick move.
There's a big difference between advising someone to power up a Sucker Punch + Dark Pulse Gengar and one with Shadow Claw + Shadow Ball or Sludge Bomb.
If it's got Shadow Claw, power it up for sure over the perfect one, because the moveset is far more important than a few more IV points.