It is mildly amusing that had Dark Empire came out now, he would be Sith and Sidious through and through. He is just the Emperor now. He is not Palpatine; not Sidious; just the Emperor. Much like Tenebrae. He was no longer Vitiate, he was just the Sith Emperor. Nothing more, nor less. Of course then he was Valkorian, and the Emperor may yet have become Snoke... To be called Darth Sidious in TLJ though; utter perfection.