How much better is salt water at letting through microwaves than pure water? 
Radio Wave Attenuation 
There are two general types of matter (substances) in the universe
  that  affect electromagnetic waves, conductors and insulators  which
  are called dielectrics by scientists. Most, but not all,  conductors
  are metals, such as copper, aluminum, silver and gold. However,  salt
  water is also a rather poor conductor! Most, but not all, dielectrics 
  are non metals. Examples of dielectrics are paper, plastic, Teflon,
  glass,  ceramic and dry wood. Pure water is a good dielectric
  substance!

Source: http://www.madsci.org/posts/archives/2002-03/1015162213.Eg.r.html
I read this paragraph, and was truly intrigued.
Is it really true that salt water (NaCl) passes radio waves (specifically 3cm) through better than pure water? Why is this, and how large is the difference? Would it be noticeable in an experiment?
 A: Salt water contains Na+ and Cl- ions. Because these ions are free to move in the solution, salt water is electrically conductive. Increased conductivity attenuates the radio waves as the freely-moveable ions absorb and reflect the wave.
(see How can I create hindrances to radio waves?)
The same principle explains why metals block radio waves and are not transparent to visible light.
Pure water is a poor conductor, as it is a covalent compound. A very small amount of water dissociates into H+ and HO- ions, but highly purified water can be 1,000,000x less conductive as seawater. 
I expect, depending on the salinity of the water and sensitivity of equipment, it would be easily noticeable in experiment. 
A: The answer to your question is "a negative amount better". You have it the wrong way around.
Quoting the post you linked to,

Dielectrics such as cardboard, paper, clear glass, Teflon, some plastics, 
  pure water and many building materials have low attenuation coefficients 
  and radio waves reflect from them and also easily pass through them. 

(emphasis added)
