I spent about 3 years in Phoenix Arizona. As a person from Sydney the heat (up to about 50 C) was just a bit of a shock.
The only cure for car A/C on cars that were post the R-12 refrigerant era was a replacement refrigerant called cooltop.
In ridiculous heat, I had 2 degree C air blowing out my vents with the fan on high.
http://www.cooltop.net/They recommend two cans for a charge, but I would recommend one to start with. Two was too much for my car over there, and the overpressure switch kept cutting in. With one can, it was just freaking awesome.
I am not affiliated with these guys at all. Just wanted to share.
Cheers,
Pete