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what matt said sounds easier. The LR will slow the flow down a bit.
But if you still want, "shaving' the impeller down sounds like you take and cut some of the impeller blades. I'd go with diagonal cutters, and trim a little off each one. I'd trim the tops, rather than the ends. If you trim the ends, it would possibly cause the impeller to wobble and wear out the housing. Just my 2C.
if the lowest setting of the ac isn't slow enough (it probably goes down to <100 gph) then restrict the intake. e.g. add a foam sleeve
the mag pumps' foam prefilter slide over the ac300/500 tube. i use it more for anemone protection but if the foam cells are dense enough (sometimes they use denser material) it slows the flow.