Due to an apparent typo in the spellscript for firestorm, it ignores the nCasterLevel cap of 20. As a result, a lvl 40 empowered firestorm would do 60d6 worth of damage and ends up doing more damage than hellball. This spellscript fixes this bug, so the damage cap for firestorm is really capped at 20d6.
The zip file includes a NCS file for your override folder and a ERF file to import into your mod.