Being a challenger owner and belonging to a Challenger group on the web I've had privy to some info on the Demon.
It may come with three keys. black key the standard 707 HP is the norm. Red Key will enable the 840HP option. Now the third key I've heard rumors about is gold.
It will supposedly unlock 1043HP. It will enable 100 Octane to be used and a tune to enable that HP and Octane use.
We shall see. since I've owned an R/T and now an SRT, it may be possible for me to think of upgrading. Again, we shall see.