Just spitballing not saying your wrong and or not partly true, but the main goal is to make the part throttle hp needed with less fuel and air, so less friction, less pumping loss so less restrictions from air inlet to exhaust tip, more cr, more efficient combustion, less heat waste, lighter rotational parts etc... Plus need less hp in the first place with less weight and aerodynamics.
Don't know if velocity gonna help much, the engine gonna have zero problem with the air requirements for the little hp needed, plus by time your done porting will there be that much difference between 920 and J heads? Heads are the number one way to make power look at LS and modern Hemis massive port flow small cam, cam choice can be one of the bigger mileage killers I'd still run better heads even if it cost slightly to mileage which I doubt would be noticeable and probably would be better cause you could run less cam for same or more hp.
