After going through most part of this thread, i tried to use one of the forummer's recommendation to use 1.6" diameter exhaust on my persona auto, the result is better low end torque, acceleration from stop is better compared to original piping which is seems like 2". For me the key is to have the exhaust velocity with less restriction. Exhaust move in pulses and the higher speed it travels inside the piping, the better scavenging effect which helps engine work lesser to push the exhaust, and also helps to induce more fresh air on the intake.
Im still monitoring my fuel consumption but i can tell you it does give me more mileage so far compared to before this