Hi Hitman,
With that setup, the extra two rear surrounds may add little to the overall sense of envelopment if you've used QS8s or QS4s as the side surrounds. Try a good 5.1 setup first.
If your room is large, adding the two surrounds at the back for 7.1 will certainly give you better surround envelopment for other listeners in different seats but in smaller rooms, the gain may be marginal at best. Of course you can always try it and see if there's a significant gain. Use a couple of stepladders to temporarily position the extra surrounds behind the couch and do some comparisons of 5.1 vs. 7.1.
Regards,
