Something I find that can happen when you’re chasing the high lift flow by messing with the SSR is………you can reach a point where getting the floor shaped better for high lift flow, ends up costing you some flow in the low/mid-lift area of the curve.
Then you have to decide which way to lean for the particular application the heads will be used for.
I would agree with your assessment of the valve sinking as being a big player in the lesser low/mid-lift numbers on the two ports..