yeah, oversizing can lead to air density build up in the wrong areas of the head. if you're talking about honda heads, they come pretty much how they should from the factory. all you should have to do to them is smooth the runners and knife edge the dividers. little changes make big differences, and big changes can mess up a head. as far as flow testing, they put the head on a flow bench and measure the air compression throughout. it helps the machinist know where choke points are in the channels and around the valves for more precise builds.