You are correct the volume is how much air it can process and not a guarantee it will do your actual space. You definitely will get overly air cycled areas and deadzones. You may find more dust settles in certain areas now. Without ducting, one thing you could try it setting up an ossicilating fan elsewhere to try to randomly kick more air into the flow of things. By doing this it will get more dust into the air at first until it can be drawn into a filter.