They had the new Viper test mule engine in there when we were there. It is a beast. I can't say any more on that.
But what isn't a secret is that they have been playing with different upgrades to the Hemi. They were working on Superchargers, Turbo's and such. They said that they do have engines that they are working on that are putting out more power then the ZL1 Camaro's... just have issues with emissions.
Look at the non-road safe Stage III Superbee as an example.