But the oil is already restricted to the heads and rockers.....in 2 places/ways.
1) The oil 'interruptor' in the cam holes having to line up for ANY oil to flow to the heads; this allows flow only about 5% of the time. And that drops at high RPM because the oil mass has to restart flowing from a dead stop each time the holes line up.
2) The smallest flow restriction is in the hole in the bottom of the rocker shaft where the oil comes up through the rocker stand; the flow area between that hole and the shaft hold down bolt is around .034 sq in.