From my experience, LSA is one badass of a vasoconstrictor. If you add the vasoconstriction from MDMA on top of that, you might have a dilemma. Or you might not.
Also, I doubt it would simply be a tuned-down candyflip. The headspace between LSA and LSD is pretty different. They both have the same "feel", but LSA makes you empty-headed whereas LSD is a big, chaotic mindfuck-thought-explosion. When I did it, LSA made me engage in all sorts of semi-mindless zombie rituals. Not that it was a shitty experience, it's just that it's not quite the same beast as an LSD trip. Having said that, combining LSA with the euphoria and otherwise increased emotional sensitivity you get with MDMA does sound like it could be gnarly, but then I'm not sure about the cloudy-headedness from MDMA being combined with it - the thing with LSA's empty-headedness is that it's crystal clear, cleaner than LSD even, which is what makes it good; yet there was this kind of vibe with my thought process that I got with both LSA and MDMA, so there could be a nice synergy there. But I could speculate about the LSA-MDMA combo forever, suppose the only way to find out for sure is to actually try it. You really gotta watch out for the vasoconstriction though, I got some bad hypothermia from LSA, to the point where my skin was turning blue and my lips were purple, but being in a cold building for several hours of the trip had a lot to do with that.