Part of imaging is also frequency response, there are a number of tweaks / cheats in speaker design that can enhance this.
What I suggest is listen to your speakers from about 2-3' away. This will give you an idea of the very best performance, with little room interaction. If they don't image to your satisfaction then, they won't do better at normal distances.
Of course, room acoustics matter, but they can't make a speaker better than it is. Assuming ideal placement, I find that acoustics in the plane you want to improve is the best approach. If you want to improve height, add treatment on the floor and ceiling, etc. Don't ignore treatment around the listening area, especially diffusion.
What I suggest is listen to your speakers from about 2-3' away. This will give you an idea of the very best performance, with little room interaction. If they don't image to your satisfaction then, they won't do better at normal distances.
Of course, room acoustics matter, but they can't make a speaker better than it is. Assuming ideal placement, I find that acoustics in the plane you want to improve is the best approach. If you want to improve height, add treatment on the floor and ceiling, etc. Don't ignore treatment around the listening area, especially diffusion.