My left/right side surrounds are mounted about 9ft up near the ceiling in my basement, rears are a bit lower, and everything sounds good. They are very forgiving on placement, and once setup correctly in the receiver settings, should sound fine. I suppose you could rig something up using the Axiom brackets and an extension to lower them a bit if needed..
M80s-VP180-QS8s-EP600-2xEP350 Denon3808 Outlaw7700
(M22-OWM22-VP100)-all in storage