Both the gasket and silicone washers have an affect on the PSU noise. There is a slight deadening to the general noise output of the unit, however, neither proved to be particularly effective in quieting the test PSU. This is because with the Silver Power (as with most PSUs I have worked with) the most noise that comes from them is not due to vibrations from the fan, but from the air turbulence created by the fan. A more effective way of lessening the noise of a PSU would be to replace it’s fan with a quieter model; however this would almost certainly void the warranty.
Out of the three options, using the gasket by itself seemed to be the most effective vibration dampening aspect of the kit. This leaves you with the silicone washers which can be used elsewhere in your case to perhaps lower the noise of a CD/DVD drive, or a case fan.
The kit costs a princely £5. Not too pricey, but definately not a cheap option considering performance.