Want to install a free flow exhaust muffler for....

[Turbonut]

my '00 Volvo S40! I am not looking for performance, but am trying to increase my gas milage to the max! Besides Syn oil/atf, max pressure low RR tires and a K&N FIPK....and learning how to drive "REAL economically." Iam looking at a universal,Walker Quiet Flow muffler sized to match my OEM muffler. This is a a free flow muffler that is highly regarded as very quiet and producing20-30% less backpressure than the average oem mufflers......so says industry experts!!!???? If I ca get a muffler with less backpressure and still be quiet and get better gas milage.....am I correct/does this make sense??? I believe in the KISS principal so I see no need to through a lot of bucks in an expensive exhaust system.........thanks in advance T/N