Chris,

I have a similar setup. 5200s across the front and 3100s in the back. My room is 13 wide by 20 deep and 8ft ceiling. At one point I had 7.1 with some old speakers and an amp.(The fronts were 5200s.) I really appreciated the envelopment of the 7.1 setup, however, when I replaced the amp and the speakers with 3100s concert videos, DTS CDs, etc sounded substantially better(even though I dropped to 5.1). The only reason I have not added 3100s to the sides is that I can not acheive that with the current M setup since I am running stereo subs. Bottom line...go 7.1 and use 3100s for back and sides.

Theater - 861v8, 621, Oppo BD83, 8K front 7200s back, 7200HC with 5200s on the sides, SW 5500
Family Room - 218, OG 8K Pair
Desktop/office system Media Source 600, 3100s with SW1600
Focal Stellia with Arche Amp and 218 for critical listening
Prime Headphone Amp and Power Supply for desktop headphone listening with Meze Empyrean