Yes long term use does increase dopamine levels and lowers serotonin levels. Like you mentioned the percentage matters, ie, 15 % wont cause effects like 99% does. two to three months max before taking maybe about the same time break.
l-dopa has been shown to reverse grey hair.
It work by lowering prolactin as well, as ive had blood work before and after on it and the only factor I changed was taking 99% l-dopa and prolactin lowered significantly.