I have noticed that the mileage is a little lower in the real extreme heat. I have not noticed any loss in performance in the heat although what would make a difference is the air density, or density altitude. A real hot day in Phoenix (1,000' above sea level) could feel like 2000' or much more to your engine. Higher altitudes can really decrease performance.

What I have found that helps when using the a/c is to use the max a/c or recirc mode. This keeps the outside air out and lets the a/c compressor work less because you are only cooling already cool air. This will help your mileage some versus using the vented setting on the a/c.

Just remember to leave the knob set in one of the vented positions and/or leave some windows cracked when you park or you will have one hot car when you get back....