You can not reduce pressure in the gear box, instead they increase the stiffness of the reaction springs, the reaction springs essentially act as the control valves as the steering wheel is turned it causes one or the other reaction spring to deflect which shifts the spool in the control valve... Full pump pressure is still used...

The shim trick reduces pressure and since pressure & flow are tied to one another the reduced volume causes a reduction in assist... It works but in the hydraulic field it's not considered the right way of reducing assist...