I don't think that is a myth. Wasn't the early recommendations to have a restricted orifice in the channels to the heads to ensure sufficient oil for mains and journals? Hemis do collect a lot of oil in the heads.

On the clearances, what has a larger effect? Crank clearances, or lifter bores? I am experiencing a similar pressure profile, but clearance on crank good, but lifter bores looser.