I have very limited knowledge on this so...., but I thought the sleeve inserts were for the stem, letting you adjust the stem angle, which would not affect the HA...
I know Specialized does have inserts for the stem to change the stem angle, but I know other companies such as Commencal include sleeves that go in the HT that change the angle.
the only thing that i can think of is that some headset companies sell spacers that go on the bottom part of your headset that will slacken you headtube angle. Like this:
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