On KRC valves, it is the venturi diameter that change with flow rating. The venturi is inline in the valve at the level of hole "A" on your image. It is not the hole seen on your image. The venturi diameter is the inside diameter when you look at the end of the valve.

Stock flow KRC 253080000 valve is 0.125" diameter for a flow of 2.91 GPM.
KRC #4 253040000 valve is 0.062" diameter for a flow of 1.66 GPM.

I don't know how you can reduce the inner diameter of the valve. Press fit a plug and drill through?

I suspect that cutting the spring might have a similar effect, reducing both flow and pressure.