It sounds like it's feasible in your space too expand to R/S. You have all ready scoped thing out, and have a mind set to do it, so you might want to try it. As to needing to be placed higher, with these speakers (QS4's) being diffused, it would be less discerning on the sound.
Question though, have you ever considered F/H speakers? According to Audyssey which claims, F/H speakers are preferable over R/S speakers. It has to do with the sound coming at us from in front vs behind us. The way the human ear is structured, audibly this would be more beneficial.
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