Originally posted by qtiger
It has more to do with the fact that most exhaust manufacturers are looking to build bolt on systems, which are usually built the same size as the exhaust manifold collector.
2.5 is a good size.
no one makes a 2.5" collector on their header in the aftermarket except the Kamikaze system (but their short primary system is anti-productive). plus, it would require a catalytic converter changeout.
a stepped system, i.e. 2.25" collector and catalytic converter combined with a 2.5" exhaust, will cause exhaust flow turbulence.
like i said, do all the research first as the aftermarket already has.