The body was carbon fiber over an altered Charger chassis by MetalCrafters (that's an oxymoron, isn't it? LOL). I'm sure the modifications made to the "altered" chassis added weight that the production car won't have (welding panels to spec adds weight, compared to one single piece fabricated to spec).
I'm sure the weight was reduced in the streamlining for production. Only Dodge and MetalCrafters know what was changed between the concept and the production fabrication process. I just can't imagine the Challenger being 4" shorter than the Charger and NOT being lighter, if not the same weight as it's bigger brother.