the issue with after market elements (those I had in my hands at least) is tolerance of manufacturing and bad manufacturing of the edges.
I.e. the edges of the control "grooves" had a radius instead of being sharp.
The reason might be wrong harness while grinding or whatever.
Next tolerances in plunger/barrel matching. Put the plunger in a defined position, fix it in an apparatus and put 350 bar test oil from the high pressure side. Measure the drop down to 250 bar in seconds. Do the same at a different plunger position. You will see that the after market elements are very different among each other. This will result in well adjustable quantity (balanced) for idle and very different quantities in upper ranges. Now you can adjust the high range but it will result in uneven idle quantities. At that point it should be clear that these elements are crap and not worth the time you spend, even when you get the elements for free.
Now you tell me that you intend to modify the control edge, good idea! but the basic quality is already out of reach.
An IP done like that will perform well but idle is horrible (did you hear that somewhere?).
This is not theory I spent a lot of money and time on that and ended up doing my own elements for a good reason.

Tom