a lot of it depends on the car, when i was in school for auto we did a few experiments trying to find what exhaust works the best for increasing power. we used a dyno to measure gains and on my car, a turbo saab, the stock exhaust worked better than open headers, in torque and power, mostly because it was a turbo and required the backpressure to make full use of the turbo.On other cars however depending on how they were setup with their intakes and fuel injection systems open headers worked better. Ganassis car because it was setup to be a racecar would have performed better with increased flow because the engine most likely has a camshaft which allows for maximum overlap and thus equals more fuel in the cylinder and more power.not to sure about this part though...but i doubt ganassis car got better mpg while he was wide open on the throttle, however if it was doing daily driving like most normal ppl do around town, he would see better mpg because an engine that breathes better is more efficient, so yes more fuel is being drawn into the cylinder, but it takes less work for the engine to create the power you need,so in the end you use less fuel.ultimately you will get the best of both worlds when you need the power it will be there, and if not you'll see gains in mileage.
pretty sure on all this, if anyone disagrees, you're wrong...just kidding, let me know what ya think
