I have a similar seating situation and I have found that going to a 6.1 system really helped with the overall envelopment. You might still want to ceiling mount the QS's, I have mine wall mounted. The rear center channel would be where the red arrow is and you would place the L side surround above or near the 1/2 wall, opposite the R. Surround you have above the sectional.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70