Not sure what 5.1 would sound like in a small room like that. I'd say you're best off getting the best 2.0 you can get. The M5s will give you the best 2.0. Kijiji the Mirage and the center and upgrade to M5.

You will never get good soundstage depth if you are FMBing or installing right against a wall.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated