"but let's assume that a customer doesn't port match the diameter of their head to the AMS manifolds -- is it a "good thing" to have manifold runners that are wider than the open chamber of the head ? What if any are the adverse impacts to flow characteristics [ turbulance and velocity ] ?"
adverse effects: yes-> due to poor transition turbulence is created which presents losses in flow(see reynolds and euler numbers). this sharp edge entrance creates flow separation and possibly vena contracta, which develop viscous effects and losses in flow.
