All I can say is I took a trip from Virginia to Daytona beach, Florida last fall. I had the girl friend along and the back loaded to the max. We drove there, a very small amount in Daytona and back to Virginia. I averaged 30.2 mpg for the trip..
The headers may help slightly on milage but GM did not install them for the same reason as most other builders of high performance cars: Higher costs, more noise and less durability versus the typical cast headers.