I received a stock B5 S4 intake manifold for flow testing that had an extrude hone performed on.  The inlet is still the size for the stock throttle body.

The Results

Below is a chart showing the flow rate through each individual runner when the other runners are blocked off.  The chart compares the extrude honed stock S4 intake manifold to an untouched stock S4 intake manifold, an IM from a B5 RS4, and a B5 RS4 intake manifold that was heavily modified.

The next chart shows the flow rate through the intake manifolds when all of the runners are open.

Comments on results:

I think the gains shown through the extrude hone are fairly substantial.  The stock RS4 intake manifold has larger runners which allow for greater flow rate at a given depression as compared to the stock S4 intake manifold, and the extrude hone brings the stock intake manifold halfway to the level that the RS4 intake manifold sits.

The RS4 intake manifold has an average flow rate increase of 27% per runner over the stock S4 intake manifold, the extrude honed S4 intake manifold has an average increase of 13% over the stock S4 intake manifold.

This photo shows the interior of the extrude honed manifold and a raw stock intake manifold.