Never been to AZ so unfortunately I can't help with the recommendation.
However, I did have a similar experience with my GS when I was in CT. As you mentioned, the Lexus dealers typically don't have their own body shops - they contract the work out. With that said, the shops they use do a lot more Lexuses than other shops (for the simple reason that the dealerships refer all their customers there), and so will have more experience with Lexus cars than your average body shop.
The other thing I found was the the majority of the cost is in the parts and not so much the paint and labor. I needed a front bumper last August and the bumper alone was over 1,000!! So, rather than going with another shop, your biggest saving will be getting a cheaper bumper. Since this generation GS is fairly new, there was no one making cheaper aftermarket parts last summer. So, then you would have to buy the bumper from Lexus and then take it to another body shop to paint and install. Given the estimates I had for painting, it made more sense just to drop it off at the dealership and have them take care of it.
So, call a couple of body shops (you'll find numbers and sometimes reviews online) and see if they can get an aftermarket bumper for you (it's been a year since I called, so they may now be available). If so, you may be able to save a bit. If not, forget the hassle, take it in to Lexus, and bite the bullet unfortunately.
Thanks man,
I did some homework and found any way I slice it it was gonna be between $800 - $1000.00 to get the rear bumper cover replaced. A Couple of places said they could apint right over the sensor, while others looked at me crossed eyed when I told them it could be done. End of the day the Lexus recommended dealers was $100.00 higher than the rest. I'll pay for the experienced shop. THanks again for responding.