It certainly is an option to get another receiver dedicated to the 5.1 speaker setup in the sunroom, and the 2 speakers wired on the outside deck (noting, I would need the ability to run either the outside or sunroom speakers independent of one another). The outdoor speakers, since there are only 2, would need to be appropriately sized, and powered, to obtain proper coverage outside. The sunroom already has a Niles HPS-4 speaker selector box in the room, would seem to be waste not to use it and replace it with a much bigger, heavier, receiver, and at additional cost.
Also, I would like the ability to link the sunroom and outdoor speakers to the Control 4 CPU located in the adjacent den. Again, this is a preference, not an absolute requirement.
Sonos seems like an interesting option, have heard GREAT things, but that the Amp only provides 55W/speaker, and given the above note regarding having only 2 outdoor speakers, I am concerned Sonos would not appropriately power the 2 outdoor deck speakers. Again, my assumption might be off-base or otherwise incorrect.