ISA + 15 flat rating doesn't make sense to me.
It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...
If the AC is overpowered, operations above 30 celsius won't become an issue, while a less powered AC won't be able to operate safely, and this, with the same flat rating value. Flat rating just gives the value at which the aircraft will suffer thrust losses, but doesn't give any clue about the AC thrust values.
Another example is the Piaggio avanti which is using 47 % of the available thermodynamical power of the PT 6 (850 shp out of 1800 thermodynamical hp), giving a flat rating of ISA + 37, meaning it won't loose power in very hot environment.