I like Poltergeist as is. Least invocations aren't very useful in general: they're either a static bonus, a low level class or racial ability (wild empathy, dark vision, scent), or a pitiful nuisance. Poltergeist fits the bill quite nicely in that it doesn't really do anything powerful, but does let you do stuff you couldn't normally do. If you want concrete power level, look to the part where you create, move, or remove any or all of the lights as a free action, anywhere within the 100'+ range. This means you can effectively light up the entire area, and since you can make any/all of the lights visible only to yourself, you can do so without anyone knowing. If you can manage to plunge the area into complete darkness against foes with no darkvision, you're effectively invisible. It's like a personal only version of the normally 3rd level Ghostlight spell, comparable to the Devil's Sight or See the Unseen invocations, Instead of seeing through Darkness or Invisibility along with your darkvision, you get ghost sound and improved mage hand. (If you do alter it, would you mind leaving the original in a spoiler at the bottom?)