The QS10s were a big difference for me because my sides are 11' away from my MLP. I was missing a lot with the QS8. I think even in your space, you will notice the difference. As for the rears, I can't hear anything in the rear 2 channels no matter what speaker I use. I traded in four QS8s for two QS10s. With the 10s, I get a more expansive sound field, more detail, lower distortion and more SPL.
Your Onk is a newer version than mine. Mine is no ADA 1500 or 1000 but for my 4200 cu. ft. space and 13' MLP, it is more than adequate for movies with M3, M5 or M100. 2.1 music is a bit different and I'll have more to say about that soon. For your room size and MLP, I'd say your Onk is more than adequate but maybe ARC can make your system sound better.