My problem with 5-HTP is this (from wikipedia):
"5-HTP raises blood serotonin levels, and because of the risks associated with elevated levels it should only be taken under the advice of a medical professional (See Chronic diseases resulting from serotonin 5-HT2B overstimulation). For instance, elevated blood serotonin levels are associated with cardiac valve failure and acute or chronic pulmonary hypertension."
Certainly taking it once in awhile seems fine, but every day seems rather risky to me.
As to it affecting MDMA... I've seen people say it helped their roll. I've seen people say it hurt their roll. I've seen people say it did nothing for their roll. Sounds rather placebo-rific to me.