Mix with high potencies for healing or damage would be fine. The balance there is in the extended execution time, not in lowering the potencies of GCD actions. If a three step mix were to result in, e.g. a 1000 potency AoE heal with a 3.5s activation sequence on a 30s cooldown, this would not be significantly overpowered compared to the other healers. 3.5s is roughly 1.5 GCDs, during which any of the other healers could do roughly 1000 potency with an oGCD and one and a half AoE spells. If anything, 1000 potency might be slightly underpowered in comparison because the 3.5s activation excludes it from being used with the same level of flexibility that the other healers have with their oGCDs.
There isn't really any reason to decrease the potency of their GCD actions for this.