I agree. Water and more surface area serve to decrease potency. How much is debatable. There's a trade off, even if it's small, between minimizing loss of active components and cooling your vapor.I would assume dry slits would have more surface area.
More surface area = more area for vapor to recondense = some level of decreased potency.
Whether it would be particularly noticeable high-wise?
Do dry diffusion slits decrease potency? I know they decrease flavor.
I notice no discernible reduction in potency whatsoever in either.
I have experienced water decreasing flavor, but never a dry piece of glass. Are you sure you are getting less flavor this way?