tire shore is how hard the tire is,
a soft tire, "lower shore" will tend to flex more (on the contct pach) and this flex will generate heat, but if the track is hot, the tire can pass its "perfect" working temp and it will loss grip.
GANSEI, on hot day you can even put 42 or 45.
Remenber as once I read on a magazine, 75% of the stting of the car is on the tires, they are the one that hace a contact with the track.
Keep testing, this will never end!!!!