RP, the positioning of your QS8s is actually more ideal for back surrounds in a 7.1 setup than for the 5.1 side surrounds which they are now. If you're going to add additional surrounds for a 7.1 setup the side surrounds should now be placed directly to the side of the listening position and should be QSs rather than direct radiators because of their closeness, among other things. The current positioning of the QS8s, as was said, would be about ideal for the back surrounds.
The advanced technology from sources such as Audyssey and Dolby which expands 5.1 sources to 7.1 or height and width speakers certainly can't be fairly described as "try to guess".
