There are no added gains with the 1/2" spacer over the 5/16, apart from an increased growl of the engine, along with the loss of your oem strut brace, as it just won't fit. The 1/2" spacer can be used for increased numbers when going FI.
In purchasing the kit, you receive everything needed for installation.
Do you need anything else? That depends if you're running a 2006 revup, where the MREV2 lower plenum is needed, as the spacer alone on a 2006 is not enough.
The spacer is a simple fit, and doesn't need any other parts or sealants for instance.