I'm guessing it'll be similar to how the 8-speed is in the Jeep GC. Put it into gear and it automatically goes into "Eco mode," i.e. slower shifts, softer throttle response, and MDS on the highway. With MDS engaged at 65 MPH on the highway in 8th I don't doubt the 5.7 will get great MPG, considering our Jeep has worse aerodynamics and can pull a solid 25-27 MPG at these speeds on flat ground. The lighter Challenger should do quite well for a car in this class.
That being said, the 8-speed is a great dual-personality transmission; pull the paddles or go into sport mode and it shifts quickly and the paddles have pretty solid feel/engagement. Should be even better on the Challenger.