Having a similar issue....I had it tucked into the right hand side of the rear view but the arrows were point the wrong way so i had to re arrange it....and i had also mounted the adhesives over the black dot area. Let me tell you it's hell getting that sticky stuff off. Tried goo gone but am reluctant to use a razor blade over the black dot area b/c I don't want to cut it.
Given the size of the rear view area on the CX-9s with the full packages there is really no "good spot" to mount it.
I did see a CX5 with an ez pass mounted on the dash, tucked behind the infotainment screen. Not sure the tolls will read it from there...I assume they will...but to do that you have to be willing to stick the adhesives onto the dash, and that will never come off cleanly (and mine is a lease care).
I am considering getting the license plate mount version. There is no ideal place on the wind shield area that is not in the passengers line of site and that is not an eyesore.