You can get a stainless steel or just a smaller mild steel back box as the standard one on the mk5 is quite big and silences it a lot. Would be cheaper than a full system.
A K&N/Pipercross/ITG panel filter makes the noise that bit nicer. I have a Pipercross in the 1.4, bought it directly from the Pipercross website a few months ago.
Another handy trick is to take out the parcel shelf

Sounds stupid but I rarely have mine in because I have gutiar amps in the boot regularly. Whenever I put the parcel shelf back in the car gets a lot quieter. Sounds silly but hey, it's free
