That made me think of an somewhat related experience on the bench with the Speedmaster head. PBR told me some months ago about the importance of flow around the shrouded side of the intake valve. So now, quite often when I make a change to the head I will flow it with the locating pins holding the head in the proper position then take the pins out and shift the head about 0.040” just to see what happens when the valve is unshrouded a little more. To my surprise, peak flow often went down when the head was moved. I finally realized that at peak flow (since the flow in my head backs up/rolls over about 0.650 lift) the air needs containment. It needs surfaces to control flow separation. By moving one of those controlling surfaces farther away, flow separated earlier.
The same could be happening with my 318 heads on the wrong bore size.