For a mirror reflection, set the diffuse_value to 0 and make sure reflection_lite_mult is unchecked. The add reflection wacro literally adds reflection - that is an existing diffuse component persists (for example polished plastic has its own color and a reflection), where a perfect mirror has virtually no diffuse color. The quality parameter of the reflection node controls the number of rays used for a reflection, so increasing it will only reduce jaggies in the reflection, if there should be some, or if you're using soft reflection, it'll also reduce grain (at the expense of longer render times).