I think you would benefit from 7.1, and the speakers you have are probably sufficient. I think several people go with the QS speakers on the side and a direct speaker in the rear.
If you feel your current speakers are lacking after getting everything hooked up, you could consider getting a pair of QS8's for the side surrounds, and then decide if you like the qs4's or m2's better at the back. Then again, with the rear speakers being up as high as the will be the downward firing QS'8s might be better than the M2's... hmmmm.
I can't comment too much since I only run 5.1 (with QS'8s), so I'll let someone else chime in.
