I am a house painter that has dealt with this type of dilema over the last 25 years,how to affect the illusion of height or exspaniveness in a space or for a given wall plane.The addition of a crown alone will do nothing unless it is used as a visual stop that contrasts to a dark ceiling plane.This defines the capping of the wall plane but if the ceiling is sufficiently contrasted the effect is one of the lid on the room having infinite depth as little light is reflected. Attention to color toning and lighting in the space is also very critical to the final effect.