>>>*When dealing with flow, remember that the goal is to have no point in the pipe act as a restriction from the manifold flange to the exit.

If the size of the cat is smaller it should be upgraded to the same size as the pipe itself. Otherwise there will be no real effect.

The reason for increasing the pipe size on exit from the muffler is to generate some directional flow, it works well.

Now if the goal is improved midrange to top end power, then a header can benefit, understand that the goal of this piece is a close to normal powerband with improvements at almost every point.

We can gain more on top end by sacrificing bottom end power and vice-versa, this would be a different article, though...*EB