Exiting exhaust under the vehicle is never a good idea because of carbon monoxide, and is illegal in most states.
If noise is what you are after on an I-5 cut off the muffler and exit strait back and down.
That gets the fumes out from under the vehicle and the noise out the back.
In today's electronic engine control I'm not so sure straight piping will do anything to performance and the drop in back pressure at the manifold may actually drop performance since the control is all about factory settings.
A re-programmer would make more sense.
Duals will add more things to drag or get damaged and the same principle applies.
On the short distance in an H3 I don't think you can dump enough more exhaust to do any good.
But hey, it's all about coming of age and wanting to "something" that generations have done for many years.
Old fuddy duddy Hunner,
but with a programmer.
This is a flash from the past before H40 on my 06 I5
Live sound
http://www.youtube.com/watch?v=_X-2hcoAKuA