Both my Pyromancer and my Support Mage have the same damage on their environmental effects.
-My Pyromancer has 28 intelligence, 5 pyrokinetic, 1 geomancer, and leaves behind a 8-9 damage fire using both Searing Daggers and Fireball, and also a 8-9 damage poison effect using Magical Poison Dart.
-My Support Mage has 23 intelligence, 1 pyrokinetic, 1 geomancer, and leaves exactly the same 8-9 damage effects on the ground using the same skills.
I tested with the same party in a previous save where they were only level 7 and they left behind 6-7 damage effects. As soon as they leveled up to 8, the damage jumped to 8-9.
Also tested with my decked out level 8 Pyromancer from another run. 44 Intelligence and 7 Pyrokinetic. Also leaves a 8-9 damage fire effect on the ground.
And my Ranger as well, using fire arrows. She has 0 points in Pyrokinetic and no extra points in Intelligence. Same result.
So skills used, Intelligence and Combat Abilities have no noticeable effect on the damage.