If you like your current waveguides, then I would move those to the ceiling and use QS8 on the side. I think the QS8 is so good on the sides compared to direct radiating that it is worth the upgrade; as Tom describes the quality is more noticeable than the number. Plus, you are saving on the rears and return shipping is low on those small speakers. If you can run the wires easy, in-ceiling is the way to go. (Not sure your budget, but the Klipsch had fantastic install ease. Cut a circle and that's pretty much it.
As for bass, I was happy with 8", and if you have in-walls already you know the drill. the sound is just not contained so it is harder to generate deep bass. Not a big problem with a good sub, but on 5 ch music, I could tell they were a bit thin. Not a huge problem as it is not a critical listening set up. I would give it a try this way. Less money out, better sound, not speakers on the back wall - win, win, win.

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire