I haven't used Axiom's wall brackets but I would have to assume that they allow for some space behind them for rear ported speakers, maybe someone who has them can contribute or give a representative a call. The 22s should also have room to 'disperse' into the listening area, so you don't want to have them too close behind you.

I doubt if you'll have any problem with your Onkyo running M80s, that receiver is quite powerful and I believe the specifications on Onkyo's site show it handling 4 ohm speakers.

Half of communication is listening. You can't listen with your mouth.