And i suppose it's well documented at what dose this is.. and of course it would be the same for everybody..
Never did I ever give specific numbers nor say it would be the same for every person. That would be silly!
While like receptors in different people will have the same cause and effect (because they are the same receptor), everyone's body will have differing thresholds as to how much stimulation (in this case: chemical stimuli) to get things going and subsequentially shut things down. (which, you and just about everybody with a clue already knows! lol)

if I still had my cell physiology textbook, I could better explain/illustrate stuff: but I sold that thing as SOON as I was done with that class.
However, it's a bit asinine to blindly dismiss any and all theories due to lack of documented proof when you're dealing with a subject that, for the most part, is not allowed to be studied.
In such a case, the best I (or just about anybody else on here) can do is to take the working knowledge they have of the mechanisms going on in the brain in a serotonin realeasing event, and compare/contrast it to what may be going on during MDMA induced brain activity.
But: once more, I agree, the specific 'dose' at which the empathogenic effects 'plateau' at will be different for different people.