The problem I had with the emperor is that is would pick at sps, More at the bases. This fish knew I didn't want it to. When I'd come by the tank it would start whistling and had a guilty look on it's face. It didn't decimate any particular coral it just pestered them and in the long-term the corals suffered for it. I sold the fish as much as I loved it, I felt it was incompatible with many of corals that I wanted to keep.
I give tremendous kudos to Gary for being successful with this fish. I was over one day and the angel sucked on the carpet anemones tentacles, I couldn't believe it. That anemone stings and eats fish like crazy yet the emp chews on it.... I guess it's an emperor for a reason... It get's its way, it's the boss.