I have known that a flush mounted speaker system in an LCR configuration has many advantages. Comparing it to a well treated room with excellent speakers and properly positioned, which will have a better stereo imaging?
The center speaker is something I never gave much though to for mixing (especially since my mixing area is so small to begin with) but I do have one for the TV surround system mainly to keep the dialog centered...and more intelligible for some of us older farts.
You mean not both flush-mounted and treated? Either or? I don't know but I'll assume that a highly-tuned room beats a single optimization every time.
Given a flush mounted room is also treated and the out into the room speakers are also tuned, which has the better stereo imaging? In other words, does a flush mounted speaker system have the depth of image (3d sound perception or others call layering of instruments)?