I just did a tire wear test.
McLaren 650S GT3 @Catalunya GP, summer, 45°C track temperature(113°F), dry sunny weather, 2pm in the afternoon. Started with 89L of fuel (default) and drove until I ran out => 37 laps.
I tried the Hard Slick for now(there is no "medium", just soft and hard) with standard tire pressure.
On the more stressed, left side of the car, the tire temperatures had an average of 95°C over the whole time with a standard deviation of 4.5°C. The track got 3°C warmer over a timespan of 68 minutes. So basically I was in the upper part of the ideal operation window.
Over the course of the 37 laps, I did not reach any real fall-off of the hard slick. Actually, had I not run out of fuel in the last corner of the 37th lap, the last lap would have been my fastest.
I could feel the wear to some degree at the end, but at that point the lower weight (89l of fuel have a weight of about 63kg) still meant that I could accelerate out of corners faster and brake later than in the beginning.
Lap 6 S1 (blue) vs Lap 36 S1 (orange):
My lap times went down by about 1s (~1% at Catalunya). Which is not entirely unrealistic.
The whole thing wasn't as useful as I thought it would be, since I never reached that fall-off on the hard slick. One lesson can be learned here though: If you pick the Hard slicks on a hot day and don't overdrive your tires, take more fuel with you than the default in GT3 cars!
I might do another test with the Softs later this week. Not sure if I should take more fuel this time, or change the default tire pressure according to the hot track surface this time. Probably be best to keep everything default and hope that I'll actually reach the fall-off wear point where lap times will get worse again and then see how fast they get how much worse.
Edit:
PS: Nice to see you back, James!