The Remean Empire is an urban legend that's been repeated so often that it's accepted as truth. It's never been mentioned in any official book that I've read (there's the odd WFRP1 book I've missed, but I can't see why it would be in any of them). I think it's a fan thing.
That said, there was a great empire in the south under Myrmidia (the principal deity of Estalia and Tilea) which originated in either Magritta or Remas (they disagree) so that could have been a Remean empire of sorts, although it's never been referred to under that name. She also only seems to have conquered Tilea and Estalia, dying at her point of triumph and with the empire falling apart after her death. Moreover, the Tileans/Estalians had very little influence north of the Grey Mountains (they're darn difficult to cross). They traded with the tribes but don't seem to have conquered them, although they did establish a trading post at Nuln.
That Remas is (still) a Roman reference/parody is undisputed (see Ricco's Republican Guard, Curious Gaesar, etc., from the 5th ed Dogs of War book) but the empire that comes with it is merely speculative.