It would open up the sound field up front, definitely, but at the price of stereo localization, which music certainly is engineered to provide, and most movies take advantage of as well.
It might sound a little more like a movie theater, where the main speakers are hard to localize and are often behind a slightly soudn diffusing screen, but I would worry about power and soudn propagation. There are no front facing drivers on the Q's and thus, the soud would have to bounce all around the room before it got to your ears. It might sound really weird.
Maybe you could try an experiment and purchase the Q's and hook them up to your theater first as your mains. Listen for a bit and see what it sound like. If it's cool, then buy the remaining two and be done with it. If it doesn't soudn good, then move them to the back and order soem M50s.
Anyone here have an easily reconfigurable system (maybe a pair of Qs on stands) that coudl be swapped out and placed up front as an experiment? Mine are strapped to the walls, and taking them down in a pain....