Not a fan of vertical builds and I can't really see it making much of a difference to the cool down time, especially as my coils are set high with plenty of air flow all around.try a single larger coil and see how it goes , any way you could mount vertical ? I would try a vertical 4mm or 4.5 with wicking only down.
Can't test your suggestion at the mo anyway because my Pheno RDA is the only one I have that will work on the mod and it's virtually impossible to build vertical on it.