My knowledge on this - I'm a graduating civil engineer and have spent the last 9 years working with cars. You're honestly telling me that exhaust will increase your efficiency so much that your mileage will increase by 26.5-35% sorry I call bs. Maybe if your cat was overly clogged, causing a cel and throwing you car into a "limp" mode which runs super rich, but take a car with no issues and you will not see that. and that would correlate into the efficiency of the engine being increased even more.
Not wanting to make this an argument, I just know that is an unrealistic increase for an engine with no issues before the exhaust.