basically, a stock exhaust is good for typical hypermilers, since most don't pull much RPM & load
now you can gain mileage with an exhaust if your normally putting a big load on the engine & pulling alot of rpm all the time
it really depends on the vehicle, how its used, etc
our mandrel bent exhausts are typically lighter then most stock exhausts, our stainless is .055" wall & our aluminized is .065 wall (16 gauge), typical muffler shop crush bent crap is 14 gauge...which is much heavier, & the crush bends alone kill exhaust gas pulse energy by constricting them down into a small area (crush bend) then expelling back out in a bigger area thru multiple bends just kills the momentum of the exhaust gas pulse
i can go on on with my ideas/theories, especially since i'm obsessed with exhausts & do i do high-end, mandrel-bent exhaust work for a living, basically that exhaust basics link on that integra site is just about right
check out our site, most of the pics in the gallery are of my work
www.mandrelbend.com