^ I've drove one of them for an extended period once... bits of trim were falling out and it had 35k miles on it. :-p
I work on cars including exhaust at work.
you won't gain anything by changing the pipes. the caravan/t&c pipes are plenty big. you MIGHT gain something from free-er flowing mufflers.
as stated, cars are designed by expert engineers...the same engineers that put half chryslers lineup battery in the d^&* wheel well and the steering rack wedged behind the engine to name a few expert designs. goal #1: cheap. goal #2: emissions (mostly manifold/cat) goal #3 of exhaust: quiet (depending on the application)goal.
the biggest FE gains are in FE oriented headers and cat followed by free flowing muffler.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"