It's not like you can just drop a supercharger on a motor and say "There ya go, have fun". well ok...you can to a certain extent but reliability becomes an issue. Without proper engineering that motor will blow itself up in no time. For starters the cam profile has to be tuned, airflow and fuel delivery has to be modified, rotating assembly has to be strengthened, etc. etc. etc. I don't believe it is any cheaper or easier to design a motor around a supercharger than to design a motor around n/a. The two companies have different philosophies and I think it's great that the debate continues.... the consumer always wins!
And for the price of a CAI and header kit that one might buy for the Challenger, a new pulley can be purchased for the Shelby that will easily give it 600hp.