actually meron but different cars would have different results from the same type of exhaust mods done.

stuff that affects performance gains in the exhaust system:
-number of joints for the header (less is better, less turbulence)
-type of headers depends on car engine & purpose for mods (daily driver or drag/race car?)
-quality of the welds (must be as smooth internally as possible)
-number of turns and curves (less is better)
-size of pipings (too large or too small)
-minimize choke points like catalytic converter, silencers, flex joints to the required minimum. If not, use hi-flow version of such like hi-flow catalytic converters, etc
-change of size of piping (exhaust gas velocity must be maintained)
-big mufflers doesn't mean its better.
-use FULL stainless mufflers. These last for years. An "aluminized" muffler will only last less than two years at most, usually about a year only before rusting out.