The boost elbow functions as an adjustable "orfice", so to speak. By tightening the set screw, you are blocking the passage way for boost pressure to get to the diaphragm. By opening the screw (loosening), you allow more pressure there, faster. It causes the WG to open faster.
If you tighten it all the way, you have essentially blocked the wastegate and it shouldnt open as long as drive pressure doesnt exceed the spring constant in the diaphragm.
Short:
Tighten, more boost.
Loosen, less boost.
You will probably notice less boost with the 14cm that with the 12cm, though.
--Jeff