The SAI (Secondary Air Intake) just warms the cat when you start the car from cold, it makes the horrendous hoover sound

You could always delete it (rip it out and add a resistor in its place

) VW wanted something like £20 per section, 3 sections from memory resistors were about £2 each and 20 minutes to rip the SAI and pipework out
You also need to either replace the bottom half of the airbox with one that doesnt have the hole for the SAI to fit into or throw a cone filter on the end of the MAF

Any questions let me know