well lets see last time I checked the motor is only a part of fuel milage. Also HP is only part of a performance equasion. It is not that we are hooking up 500 horses to our cars. There is also torque to move the mass of our cars.
Give me the Torque of a Pontiac, why was the RA 4 GTO so close to the LS6 Chevelle in the quarter with 84hp lower motor?
I personally feel Chevy motors are used because people really don't do there home work. Plain and simple. I bet a Pontiac could do what you want and it would just be harder then going to the local K-mart and looking for that bluelight special on Chevy motors.
That is just my opinion though. So it all comes down to cost yet again.