Thinking about this thread, I wanted to point out that the biggest improvement I got was going to shorty headers. I bought Gibson ceramic coated stainless ones and got 10% better gas mileage (10 to 12 so big whoop. Also, throttle response without even replacing the rest of the exhaust. When I added the Jet chip I needed to run premium gas, but felt the off throttle response even more. Just my $.02 from my experience.