Yes it's true, using "Z-Splash" will increase Attack by 3 stages (2.5x usual power).
It's good, but it seems unlikely to become "viable" IMO. You can only use one Z-Move per battle, so if you switch out and back in, your boost will be lost. Z-Moves also require holding the Z-Crystal, so you can't hold other items. You're probably better off using a Choice item.
I guess it's possible that Magikarp or Hoppip could see use in Little Cup, but that's about it.