The "flow" of your motor is its ability to get a gas/air charge into the motor, combust it and then get the spent exhaust gases out. Anywhere that process is restricted robs your motor of efficiency. Exhausts often restrict flow.
Yes. Exhaust systems are usually designed and marketed to increase the power of your motorcycle. But power and fuel efficiency are not mutually exclusive.
We think of powerful motors as getting bad mileage. But if you help a motor to make more power by "breathing" better and you don't use that power to go faster or quicker, the more powerful motor will do more with less fuel.
