As I understand it, oral 5-HTP supplementation is not a major concern for serotonin syndrome: it is mostly degraded in the gut, whatever gets absorbed has a large fraction turned into serotonin outside the blood-brain barrier, and rapidly destroyed by MAO.
If you can find a single case of 5-HTP being the causative agent of serotonin syndrome, I'll eat my hat. Almost always, SS is caused by multiple serotonin reuptake inhibitors/releasers taken in overdose, and quite often some drug that ptevents serotonin from being degraded (MAOIs).
A review by Juhl found no reported human cases of serotonin syndrome induced by 5-HTP(ref)
Serotonin syndrome is determined not only by drug doses, but also non-drug factors. MDMA causes a dose-dependent increase in 5-HT, activating extrasynaptic 5-HT receptors producing serotonin syndrome. However, what receptor subtypes (especially low-affinity 5-HT2A receptors) activated will depend mainly on the receptor responsivity influenced by non-drug factors (ambient temperature and physical activity). Cold ambient temperature diminishes responsivity of 5-HT receptors to high 5-HT, resulting in a mild syndrome, while, hot ambient temperature alone (1× factor), or in combination with physical activity (2× factors) increases responsivity of 5-HT2A receptors to high 5-HT, resulting in moderate, or severe and life-threatening syndromes, respectively
(ref)
This one study actually
claims that 5-HTP and tryptophan reduce neurotoxic effects of MDMA, not increase them.
In fact, didn't people used to "pre-load" by dosing 5-HTP a day or so before a MDMA experience?
Erowid has some experiences with 5-HTP in combination with MD(M)A. There's nothing too conclusive, some people claim to have stronger trips, some people claim it does nothing much, some people claim it makes them crazy intense. One even claims that 5-HTP brought the magic back to MDMA.
Compare levodopa (L-DOPA), the analogous amino acid precursor for dopamine/norepinephrine: it doesn't cause hypertension on its own, and given for Parkinsons, you need to take truly outrageous amounts to have an effect if you don't also take drugs to supress the metabolism in the body.
TL;DR: As long as you aren't taking like, 5 grams of 5-HTP every day, it's not anything much to worry about IMO.