with a solid bushing i believe it would eliminate movement of the rack itself period not necessarily vibrations coming from the rack but again the rack itself from moving around in its brackets. with softer materials the rack still gets movement where over time the rubber or poly can deteriorate. my two cents, feel free to correct me.

I replaced my steering racks with Daizen on my SC, it eliminated vibrations because my wheel and rack werent bouncing around anymore inside the bracket when i drove.