Yes ghost riders needs now 6 to cast. Also keep in mind the doubled research costs. The need of 6 to cast ghost riders forces you basically to use demiliches to cast ghost riders.
At least most nations. Because most of those national mages that can get death 4 have also old age and thus need to get replaced every now and then.
For fire there are now to quite useful looking new battlefield spells
.
Evocation 8: Pillar of fire. 2F to cast.
Range 100, AoE 1.
34+ ap damage.
Alteration 8: Conflagration. 2f to cast.
Range 25, AoE 10.
Precision 100
Works like high area combustion. Armor is ignored this way. So a good spell to get rid of highly armored human troops.
These 2 spells give fire even more flexibility on the battlefield, and f2 is not a high requirement.
And then hellswords and soul contracts need now some fire gems too for forging.
And finally fever fetishes got thematically nerved, they do not produce fire gems every season now but rather unpredictable, so every other season only.
These changes should make fire magic better than in dom2
.
I think fire might be better battlemagic than air in dom3.