About henna drying out your hair, my hair seems not to be dry at all, except for the ends which were colour treated previously and quite damaged to begin with.

It seems that the strengthening/thickening action on the hair shaft is multiplied too much with all that porosity for the henna to stick to. The parts that are damaged accept too much henna, making it brittle and dry.

I would imagine if your hair is naturally super porous it would not be a good candidate for henna.