Because you sit close to the QS8s, and surround is 1/4 of the total content, I would say you would get more benefit from upgrading the fronts. I on the other hand sit 12 feet away from each QS8 so I am anticipating the QS10s will make a big difference.

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