How to lower the opacity of the alpha layer in an svg filter?

One way is to add a feComponentTransfer filter primitive, like this:

<filter id="dropshadow">
  <feGaussianBlur in="SourceAlpha" stdDeviation="3"/> 
  <feOffset dx="2" dy="2"/>
  <feComponentTransfer>
    <feFuncA type="linear" slope="0.2"/>
  </feComponentTransfer>
  <feMerge> 
    <feMergeNode/>
    <feMergeNode in="SourceGraphic"/> 
  </feMerge>
</filter>

A live example can be seen here.