WHM already has that long cast time on Holy. If the other healers get effects on them they should be around the caster and have 2.5s cast time. But honestly, who would want them all to be the same?

What I would change is make Gravity's AoE just slightly bigger so it'll hit more than 1-2 enemies if they're not grouped up perfectly, and make Art of War and Dyskrasia different from each other. They're practically the same now.