Jeez! I thought that some of you knowledgeable people would have had something to say!
My conclusion at the moment is that the direct oiling of the rollers on the lifters is causing more oil to flow and hence the pressure drop and/or that the shaft rocker system is flowing more oil and causing the drop. I do not think that fitting std type restrictors will be good even though it will raise the oil pressure before them in the system it will lower pressure after them hence reducing oiling to lifter rollers which are known to suffer on street engines. Next maybe I should consider restrictive pushrods?