I just bought a Onkyo 515 to replace an Onkyo sr 302 that is a 6.1 The 515 works with 6.1 and I would bet it would be a lot easer to add a rear center. My room about the same with glass doors and fire place on one side and a 6 foot open entry on the other, with no place for sides to go. The rear center does give a strong placement to the sound but I do not see a lot of dolby ex or dts ex on much of anything, so the AVR most of the time is creating the center back or with 7.1 the surround backs from 5.1. There is also an option to have high fronts rather then the backs.
I think that if I did not have the 6.1 and knowing what I know now I would just stick with the 5.1. I went to the new Onkyo for the lossless audio and the HDMI switching rather the extra channels. I am using Polk Audio's RM 7200 system from 10 years ago with very good results.