Although a reed spacer will add no power, they do move the powerband up or down, depending where it is inserted.
If between the reeds and the jug, the powerband comes on earlier.
If between the reeds and the carb boot, the powerband comes on later.
It is a lot of money for a little block of metal but if you want to shift the powerband it does work.
My lad uses one and the powerband comes on strong in the low region.