The SIII covers do flow worse because they changed the type of casting when Series III's came out, due to the maintance costs to keep the old casting\machines going. The oil passages are much rougher and do no flow as well. Read this: 3800 OILING -TECH- analyzing deficiencies - Page 6 - 3800Pro.com Forum

As far as the head's, I agree. The intake valve is a whooping .03" bigger, I'm sure it adds at least 350 CFM to the flow (just kidding).