OMG, bling bling.
20" is going to be nice on your CX-9.
Couldn't pay me to put anything bigger than 18s on our '9. smoother ride, and don't have the ridiculous looking non-existent sidewall.
Bigger wheels don't improve performance. It's a look, nothing else. In fact, they reduce performance as well as ride comfort.
All the complaints about ride are GT owners. That's enough for me.