G, not sure that I have the full picture, but apparently you're talking about the side surrounds in a 5.1 setup. If so, the QS8s, mounted at the height you mention and preferably a couple feet farther back on the sidewall than the listening position would be advisable. For side surrounds the very wide dispersion of the QSs would make them preferable. If you were talking about also having back surround speakers in a 7.1 setup on the back wall, a case could be made for M2s instead of the QSs, but that apparently isn't your plan. Unless the back wall would be very close behind the listening position side surrounds shouldn't be mounted there, because they're primarily for side, rather than back effects.
