My experience is that with that much distance behind you direct radiating speakers will likely work better. In my rooms I’ve found the sweet spot for using QS8s as rear speakers is about 5-7 feet however, I’m sure that is also room (i.e. wall ceiling seating etc. . .) dependent. The only way you will know is to compare them.
Also IMO take general recommendations from Dolby, DTS and Audyssey with a grain of salt. Their recommendations are a good starting point but what really works best for a given situation is very dependent.
I’ve been considering trying out my M2s again on the back wall because with the depth of my room the panning of destrete sounds like voices in video games isn’t moving as smoothly across the back as around the rest of the room. They don’t quite jump from the left to the right side but the pan across the back is pretty quick. However, it might be just as simple as bumping up the sound level of the rear speakers a little.
The one problem I do find with using direct radiating speakers as rears is the its difficult to tell if a sound that’s supposed to be directly behind you actually is and not in front of you. When something pans like in a video game that’s not a problem because you hear it moving across the back OTOH if the sound starts there or stays there it’s hard to tell it’s behind you and not directly in front.
