The hotter the track the harder the insert you should use.
We had someone who got good times on RP24's with hard inserts when the track was at 45 degC.
Try some and see what happens.
A big air gap usually helps the tyres to heat because of the extra movement they allow.
So try to find some hard inserts with virtually no air gap.
The higher temps will also reduce the efficiency of the motors, so it porbably isn't all down to the tyres.
Skiddins