I have a room (10 ft x 10 ft with a small alcove for the door) that images perfectly in 2.1 with a sub.
It's a near field setup and you definitely have to tune your setup to optimize it.
I used REW and the brethren here to help me get it dialed in.
It is freakin' outstanding. Not just good. Its outstanding.
I think for anything greater than 2.1 you are in the land of diminishing returns. You can't get your rear channels behind you to any real extent. It is possible to make it work. I know you can make 2.1 work just great.
If after you've perfected that, you still want those extra channels, you have a great baseline to work from.