The distance between my outside surrounds is 25 feet. I notice very little difference with my two rear QS8s. Maybe I'd notice a bigger difference if I had direct radiators in the back like M22s or M3s. Disclaimer: I have a very live room.

The best suggestion I can make is put the wiring in and try it for yourself.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated