If i do a cat back exhaust system will i get better fuel economy ? How much better?

Cat Backs dont help Gas Milage. Although they dont hurt your Gas Milage by much. Maybe the MAX of 1 mpg.

no silly... u will have worst fuel economy.. think this way... u can put out more fumes fast.. that means u can burn more gas so that means u have more power (well this is the simple way of putting it)