I got a 1.5" emp and put it into my 90G sps after isolation. It grew fast and I moved it to my 210 about 3-4 months later. It never picked on any SPS but did eat the few straggling zoos/polyps that I had.
However, a local hobbists had an emp from the same LFS order that would not leave his SPS alone... or clams.
I would say that it is 50/50 and not to try one unless you can easily catch it.