It doesn't open it up very much. Maybe a 1/6". Mostly it removes casting irregularities than bump up from the surface and smooths it out.

On a Hemi, the manifolds are about as good as you'll find anywhere. Extrude hone might be more effective on worse manifolds.

In the original Magnum V8 press release Chrysler specifically brags about
“new Balanced Flow” Magnum exhaust manifold design.

If the exhaust flow varies between cylinders
the intake charge filling to the cylinder will vary
as will the charge temperature.

It is not just peak exhaust flow, unless it is a one cylinder engine.

Waste of money for the most part.