I experienced the heat cycling out issue this weekend, and it happened fairly quickly. We had a three race weekend. Won the first two, and by the 3rd one, couldn't keep the same guy I beat in sight the whole race. He ran the same lap times, I was off by 1.5 seconds. No set up changes. Because the tires came with the car, I'm not sure exactly how many HC were on them (some of them were also in the rain). They still had a lot of rubber on them, however.
Joe, your thought of setting up the car to understeer more is a good one, if less fun. I came in with rear pressures up about 2 lbs over the fronts, so I was overheating the rears. Wear was even, and I'm running the same camber setting as you.
One interesting tidbit is that wear was more even on this car than my last one. Alignment settings were the same, but the current car has much stiffer sway bars (Tarret vs. Turbo), and a late offset suspension. I suspect this means less dynamic camber loss in corners, leading to better wear. Something to consider - softer cars, especially those with higher ride heights, may need more static camber than stiffer cars.