It occurs to me that I HOPE that they DON'T write stats for Horus. There are some things that should just be left behind the curtain. Horus fought the Emperor within an inch of his life. That tells me that Horus is nearly as powerful as the Emperor... i.e., if we have Horus' stats we also pretty much know the Emperor's power level.
Once you have stat lines you start being able to mathhammer the mystery and joy out of the central storyline of the 40K universe: "I could kill the Emperor with X number of Carnifexes or Y number of Storm Ravens." It kills the mystery.