Want to Install Free flow muffler on my S40. I need..

[Turbonut]

feedback please!!!!!!!!!!!!!!!!!!!!!!!!! I am contemplating this mod to increase my MPG!!!!!! I am looking at a Walker Quiet Flow universal free flow muffler sized the same size as my S40 muffler! This particular muffler has a reputation as being very quiet and producing 20-30% less back pressure!!!!! So says the experts1 This is my problem, I am being told that if I reduce the back pressure too much...I will lose HP and MPG, when I want to gain.....Are there any Bricks out there with any muffler mods along this line????? And what do you think of this idea? My guess is that I am helping the MPG not hurting it1 I subscribe to the Kiss principal....so I am not going to through a lot of $$$$$$$ into some elaborate muffler system! Appreciate all the muffler related replies I can get, thanks in advance. TN